Request: support for external images

[ZiNgA BuRgA]

Checking the file once does not guarantee that it'll be valid forever.
Especially if you're trying to verify dimensions - it's trivial for a server admin to replace a small image with a larger one.

Thus verifying an arbitrary URL seems mostly pointless to me.  Perhaps the only thing I can think of is perhaps tell a clueless user that they need to supply an image link, though requring a .png/.jpg/whatever extension is probably suffice most of the time, and of course, the MyBB [img] tag does no such verification anyway.