Easiest thing I have found is to create a user name for photobucket.com. It's a really simple to use site. You upload your pics from your computer into your photobucket album. Once you have uploaded your pics, go to your album to view them. Under each pic is an "img code". You click it to copy it, then you paste the code here on the forum.
One other thing to consider is resizing your pics. Most cameras take enormous pics. Downsizing them a little is nice.
I think there is also a sticky around here, maybe it the Rate My Tank section, that has tips and directions for posting pics.
Good luck!