If your file is over 75KB (and therefore can't be uploaded as an attachment to a message), best bet is to open a photobucket account (doesn't take long). Then you can upload images to photobucket and insert the images into you posts within the [im
g][/img] tags.
Like so:
[i
mg]http://i219.photobucket.com/albums/cc43/kinboshi/england-flag-65.gif[/img] will give you: