Imag Links Revert To Deault URLs

Hi,

When I try to edit the link URL of an image in a post, using the standard method (hovering over it and entering a URL in the pop-up window), the URL shows up correct in the HTML source, but then if I save or preview the post, the URL reverts back to the default (the URL that points to the image itself). This happens ever time I edit an image URL and save the post. Please advise!

Justin