Charles,
If you are asking about the size of various pic formats that you upload as an attachment, there is a size chart just below the upload window. When you post a reply, go to the Advanced, towards the bottom is a button for "Manage Attachments". Clicking on it brings up the upload window.
If you are talking about pics stored off site, there isn't really a limit...just the visual size to make viewing easy, but not so large you have to scroll to see the full pic.
Does this answer your question?