Since most of the popular social networks here below only use square images – cropping rectangular images when necessary – and they all show your image at various sizes, there are only really 2 ways of going about this:
Submit a good quality, large image that still looks good even when reduced in size, or…
Select an image according to which size is most likely to be seen for a given social network
I recommend the first strategy. It's just easier to apply because all you need to remember is to reuse the same single, good quality, large image of at least 180 x 180 pixels whenever you're asked to submit a profile picture for any of the social networks listed.
Use this first strategy with one condition; for the social networks where you spend most of your social networking time (like I currently do with Twitter), use an image tailored to that site's guidelines and/or restrictions, which is where my recommendations come from.
Some of the recommendations haven't changed since 2009, some have, and I've also included newly-popular networks in the list.
Recommendations for your social media profiles in 2010
Facebook’s profile page lets you display up to a rectangular image 180 pixels wide x 540 pixels tall while showing avatars of 50×50 pixels elsewhere. Recommendation: 180 × 540 pixels. If you're not happy with how Facebook crops your image in making the 50 x 50 avatar, use a 180 x 180 pixel image for your profile.
LinkedIn’s sidebar images measure 40 × 40 pixels and profile page images can be 80×80 pixels. Recommendation: 80 × 80 pixels
Twitter profile pages display images at 73 × 73 pixels while showing avatars at 48 × 48, 31 × 31 and 24 × 24 pixels elsewhere. Recommendation: 48 × 48 pixels if you put a larger image as part of your Twitter background, otherwise 73 × 73 pixels.
Although Google’s Gmail (96 × 96 pixels) and Friend Connect (both 82 × 82 and 40 × 40 pixels) use different images, the image sizes aren’t too different. Recommendation: 96×96 pixels
Foursquare is “a mobile application that makes cities easier to use and more interesting to explore” or, a social network related to places & brands. Lets you display a square images showing avatars of 40 x 40, 30 x 30 and 110 x 110 pixels on your profile page which is limited to “200 KB or smaller (jpg, gif, or png format)”. Recommendation: 110 x 110 pixels
Gowalla is a location-based mobile social network that inspires people to share their favorite places with friends around the world. They call their avatars “icons” and say “Should be square, preferably 100 x 100 pixels, and in JPEG, GIF, or PNG format.” Elsewhere on the site, your image will be displayed at 30 x 30 pixels. Recommendation: 100 x 100 pixels
Digg v4, released in August 2010, emphasizes 2 image sizes: the profile page can display an image up to 140×140 pixels, while the rest of the site shows mostly shrunken versions at 24×24 pixels. Images can be no larger than 5 MB. Recommendation: use an image of 140×140 pixels that is still clear & recognizable at 24×24. Otherwise, upload a better 24×24 pixel image since that will be seen much more often.
Yahoo! Pulse is Yahoo's Facebook-like site. The Profile page has your square image at 128 x 128 pixels, but the most-used size is the reduced 32 x 32 that appears near comments. Recommendation: 128 x 128 pixels
Like Facebook, Skype is one place where rectangular images can be used, but that doesn't mean you should. Both 96×96 pixel and 48×48 pixel images are used on Skype. Recommendation: prioritize the chat window by submitting an image of 96×96 pixels.
Things have changed since Ning became a paid website… Ning will take your GIF, JPG or PNG file and display it at 115 x 115 on your profile page, and elsewhere at 40 x 40. Limit of 10 MB. Recommendation: 115 x 115 pixels
FriendFeed is like Google in that the different avatar sizes used aren’t too different at 75 × 75, 50 × 50 and 25 × 25 pixels. Recommendation: 75 × 75 pixels
Gravatars are universally-used for blog comments, like here below. Although the Gravatar website will preview your avatar for you at 192 × 192 and 80 × 80 pixels, webmasters can choose a size that fits their design and that is usually something smaller, closer to 50 × 50 pixels. Recommendation: 50 × 50 pixels.
Also used here on Personal Branding Blog, MyBlogLog is a great way to see who is visiting a site. Files must be in JPG or PNG format, 5MB or under but will be reduced for the avatars to appear at 96 × 96 pixels, 48 × 48 pixels and even 24 × 24 pixels on some sites. Recommendation: 48×48 pixels
BlogCatalog is like MyBlogLog and almost as useful. Making things simple, there’s only one possible size here for your GIF or JPG avatar image. Recommendation: 50 × 50 pixels.
Myspace allows up to 158 × 158 pixels for an image on the profile page, and uses 28 × 28 pixel icons everywhere else. Recommendation: 158 × 158 pixels
StumbleUpon lets you use a large 185 × 185 image on your profile page but will settle for any size you submit, while using avatars of 48 × 48 pixels everywhere else. Recommendation: 48 × 48 pixels.
A blogging social network. Add an image to your site that is 170 x 170 pixels or smaller to achieve the best results. Larger images may be compressed automatically, resulting in image distortion. Unfortunately, many users have had trouble in 2010 adding profile pictures and Xanga hasn't been too responsive in their own forums. Comments avatars can be a maximum of 60 x 90 pixels. Recommendation: 170 x 170 pixels
Google's social network also announced some changes in summer 2010 with a recommendation to use square profile images. Orkut profile pages display images at 96 × 96, 66 x 66 pixels, 60 x 80 (in the Communities) and 35 x 35 for the friend avatar. Recommendation: 96 × 96 pixels
Friendster was considered the top online social networking service until around April 2004, when it was overtaken by MySpace. Photos must be no larger than 2 MB. Profile images are square, and appear across the site in sizes of 35 x 35, 40 x 40, 130 x 130 and 200 x 200 on your Profile page. Recommendation: 200 x 200 pixels
hi5 is a social gaming network, open to people 13 and older. You can't upload photos that are more than 2 MB. It lets you display square images showing avatars of 100 x 100 pixels, 75 x 75 and 50 x 50. Recommendation: 100 x 100 pixels
A handy tool for avatar-making is: mypictr
Still the easiest tool to create & resize your avatar image online.
From my 2009 guide: “With mypictr, upload a large avatar image and choose the destined website or social network. mypictr then suggests the default avatar size for the chosen website and lets you zoom and crop your uploaded image until you have a match. Once you click ‘pictrit’, you can download your image or email it to someone else.”