I've created a module which allows a user to force images to be converted to a specified format. The reasoning behind this is that we want to allow our users to upload images of any format, but for them to be viewable in a browser. I fear there may already be an easy way of doing this, but I couldn't find any option to do it.
The module provides an extra option "ImageMagick Convert All Files To" on the "image toolkit" admin page when using ImageMagick. Currently you can specify that all images be converted to BMP, GIF, PNG or JPEG.
Unfortunately I've had to patch the image.imagemagick.inc file to allow for this, although this change is very minor.