Using SimpleTest to test an image upload
There is a base image field test class in image.module, which has a uploadNodeImage() method. You can either re-use that by extending from that class or steal the code there.

See testStyleReplacment() for an example of how it is used. 

Edit: The "Upload" is optional and only necessary if you want to upload multiple files. You can always just save the node directly and any selected files will be uploaded and processed.