I had that problem with the "Upload Picture" feature the other day. I think it's a problem with the horseforum.com site. If you have the photo uploaded elsewhere, you can just click the IMG button and then type in the UPL (and end with the IMG button).
I had some trouble the other day uploading a photo. I use photobucket.com for resizing and such for posting on web sites. If you decide to use them you can just upload your photos to them and then click on the IMG code and it will copy it. Then just paste the link into the body of your message and it will be posted. I try to resize them to email size otherwise they will post huge on the this site. The thumbnail size works best on here for the avitar. Hope this helps.