Not sure if you managed to get this to work, or just having problems with uploading images.
At this moment I'm also trying to come up with a way to use the ImageResizer with the default Sitefinity providers.
For now you can make use of the ImageResizer by creating your own provider. When you create a library that uses this provider, the images are stored on disk, so the ImageResizer can actually do it's thing. I have this working and was planning to blog about it.
I can post the instructions later today. In the mean time I really want this to work also with the default providers, but I'm still working on this.