The fact that you can see when any given userpic was uploaded in http://www.livejournal.com/manage/files.bml?showtype=userpic (and its dimensions and size) seems not to be mentioned in the FAQ, as ldymusyc pointed out in suggestions.

It seems like this is something that should get mentioned, but I'm not sure which FAQ fits it best.

http://www.livejournal.com/support/faqbrowse.bml?faqcat=userpics

1 sounds like a reasonable candidate. http://www.livejournal.com/support/faqbrowse.bml?faqid=1
46 is also a possibility, but I don't see an easy way to make a connection. http://www.livejournal.com/support/faqbrowse.bml?faqid=46