Ok, setting the thumbnail on BISO files doesn't work right now, so I made a simple tool to set it outside of PE.
It also allows you to replace one material with another, e.g. if you want to use the Protoss Colony Pack but don't have enough gold
(it REALLY ugly. I'm a web developer, not a web designer :/ )
Hope I'm not braking any TOS or anything with this, if so I'll take it down