alright, so i'll break this down for you and explain it all and whatnot
http://steamcommunity.com/id/razputin005 <-- my steam community link
http://steamcommunity.com/id/razputin005/screenshots/ <-- screenshots folder, note it's the same URL with /screenshots/ appended
http://steamcommunity.com/id/razputin005/screenshot/594691397341237095 <-- link to picture, appended picture ID or whatever
http://cloud.steampowered.com/ugc/594691397341237095/164C2822464A074EF8E464B7F2E98D41B58E65B0/ <-- direct link to picture, you can get this by right clicking the picture in above link and selecting "view image" or clicking "view full image" to the right of the picture on the steam page
if that's the exact size you want it, you'd be good, nbd. however, you'll likely have to save that to your computer somewhere, and crop it using something like GIMP in order to size it for an avatar/signature
or, you can crawl around in your steam files, and you'll eventually find the screenshots folder, but uploading them to the steam cloud is a bit easier for me, meh.
also, you can select to use that screenshot "library" window thing steam has, and then view on disk or whatever. not sure how easy that is or exactly how, but it's there.