It's really very simple:
Read our
Posting Guidelines, which sets a limit for 600 x 600 pixels. A link to those Guidelines is located at the very bottom of every page! Those limits were set by the owner of this site, by the way.
The easiest method to post an image is to go to '
Manage Attachments' and (after you finished writing your post and prior to posting it), simply upload the image!
CHEERS!
