I'm afraid that Moxnix has the right answer - re-name the file. It's a pain in the keester, and I have to do it now and then, too. It's something that's part and parcel of the Forum's software and I don't know a way to get around it.
I try to pay attention, as I'm getting a photo or file ready to post, to the file's size. If it's anywhere near the system's limit of 500k bytes I put the thing up on the Photobucket site, where size doesn't matter, and then use their link to our site/Forum here. it's another pain -- but at least it works the first time. As for re-naming the file (see paragraph 1) -- remember that the computer is kinda dumb - that re-naming might consist of changing or adding just one letter or digit. That is, "AZgearhead" might become "AZgearhead1".
So you said you tried four times without luck. Two of 'em were the photo/size/name thing -- were there more hassles? Lemme know - via PM if there's something you want to put on the Forum as a surprise or whatever.