If it says that url is invalid, than it's probably truth. Many people post here every day, so somebody would notice if there is some problem.
Try this:
1.) go to
ImageShack® - Online Photo and Video Hosting2.) choose photo you want to upload (button Browse)
3.) "Upload now" with the setting Image resize to 800x600 (15-inch monitor)
When the process is over, you'll see your image and under your photo you'll see "Links to share your image:". Pick the one called "Forum Code" and post it here.
That should do it