Following on from Chad Crowell's answer: I would say that the easiest way to fix this -- assuming you're connecting to a single UNC server -- would be to map the UNC path to a drive letter. This is really easy, in Windows -- just open an Explorer window and go to Tools -> Map Network Drive. Enter \server\images in the obvious box, and pick a drive letter in the other obvious box. If the server uses different credentials than the Windows login, then check the "Use different credentials" box. Then click "OK". (If you've checked 'use different credentials', it will prompt you for those after you've clicked OK; there will be an option to save them.
Then you can reference the paths from Imagesizer as, say, Q:\media (maps to \server\images\media) or Q:\stuff (maps to \server\images\stuff).
Failing that, use CE Image. :)