I’m more than slightly computer illiterate myself, but figured out how to do it using Photobucket (I didn’t try the other sites, so I can’t say whether Photobucket is easier or harder, just it was able to be used pretty easily by me). When you get the picture onto your computer, Photobucket can sort of grab it from there. Once you’ve downloaded it into photobucket, there’s a button you can click to edit the photo. Click that and then the ‘resize’ button, and change the size to ‘messageboard’ (took me awhile to figure out that if ‘messageboard’ wasn’t a size option, it’s already that size). Once you’ve done that, it’s ready to use. Copy and paste the bottom address under the pic- it’ll be something like [IMG]…./[IMG]. If you click on the right spot on that line, it’ll copy for you. Then come to your post-in-progress here, right click in your post, and click ‘paste’. That should put the address-thingy in your post, and when you submit, the picture will be there.
Sorry this is so long- if you’re as bad as I am, it took me forever to figure that out- so hope it helps!