The generic file icons - like the Excel one - are embedded as resource files in the project TIS.GSP.Business, so to change it you'll need to update the source code and recompile. I hope to make this easier in the future.

As far as the quality, I could not find a higher quality image on my system, even though I could for the other Office file types. If you find one, send it to me and I'll include it in the next release.