You know.... An easy way to do this, is by uploading the file to the imageset folder, and then going to the ACP > Styles > Imageset. Change is there. If you do replace the current logo, then go set the size to both be blank. It will automatically find the size. No need for editing imageset.cfg.
P.S. I know you have already completeled what you were trying to do, I am just showing you an easier way next time you want to do it