The problems you are encountering with Sitefinity 5 and IIS6 are actually known issues, which you can fix by adding entries for the different static files in your httpHandlers section of the webconfig. For example if you want to have the .png files served, you will have to add:
You can also add entries for .css, .jpg, .swf and other static files.
the Telerik team
Do you want to have your say in the Sitefinity development roadmap? Do you want to know when a feature you requested is added or when a bug fixed? Explore the Telerik Public Issue Tracking
system and vote to affect the priority of the items