The images you use as forum icons are submitted as a file-path. So this function doesn't have a dedicated upload feature.
What you end up with is that you will have to serve the image that is already on your server (uploaded via FTP or whatever), so you can't resize the actual file anymore. You're left with 2 options.
1) Reduce the size of the image with CSS (by setting a max-width/height)
2) Upload a smaller version of the image, and use that instead
Option 2 is way easier (and saves you bandwith). So just upload a smaller version