Do you have 10 rep points yet?
I believe people start out with ten rep points by default.
"error writing temporary file" doesn't sound like it has anything to do with AW. If your pic weren't meeting the requisites, then you'd have seen a message indicating something more specific about the dimensions or the size of the file.
This sounds like it could be a caching or buffering error - that there's a problem copying the file to a staging area while it does the upload operation.
It might be that you've run out of temporary file space on your computer, (or that you don't have access to it, which would be weird). Internet Explorer, for example, allows the user to set the maximum disk space to use for storing temporary files [Tools->Internet Options...->Settings...], as I'd expect all browsers would.
Perhaps try clearing your browser cache, temporary files, cookies, etc. from your computer. In IE, for example - or at least the antiquated version I'm on now, select Tools-> Internet Options... and select the "Delete Files..." button.