Actually, resolution is irrelevant. But I would suggest from a PC one uses the .JPG format, and there's a 2Mb cap on files (IIRC).
However, there is also a limit on the server space available to users for their images - which increases as your post count increases (http://www.whatdigitalcamera.com/for...q_memberlevels).
As with all images uploaded to the Internet; best practice is to optimise before doing so - I would also suggest that anything larger than 1920x1080 is probably wasted in terms of online viewing at this point in time, and mean monitor resolutions.
Clearly, uploading for printing is another kettle of frogs.
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