I feel your pain, Sanford, as to date I have been unable to upload an image from my computer to a forum thread. Have cleared cookies to no avail. Crazy part is I can upload images directly to my portfolio, and can upload images from Flickr to a Pnet thread, but when I try to do that from my computer, all I get is an error message.
By the way, these are known good images that fit the Pnet size requirements, and had been uploaded successfully previously with the old forum software. Very frustrating! Using a Windows laptop, running Vista Home Premium and an up to date Firefox browser - which is the exact same configuration used with the previous forum software. Go figure. :(