Dude, I just typed an F'n step-by-step manual on how to upload pics and the damn window on TT closed on me... I'm not doing it again.
Quick version: First order of business is to put your photos on a photo-hosting site, I use Photobucket. Then just copy the IMG or HTML code that is associated with the pic on Photobucket (or whatever), now paste that code in your TT reply. Should work.
If you have a nice camera with great resolution, the file size might be too big. You'll need to "dumb down" your photos by changing the file size to something smaller.