JoshQ

CAS thumbnails require three things inside a package, besides the actual image: a ‘type’, a ‘group’ and an ‘instance’; you can check all these using s3pe. 

For the first there’s a list depending on the type of clothe / object you’re doing and thumbnail size, generally speaking, you only need the biggest size (256x256 png) for garments, shoes, accessories and hair, all these use ‘THUM 0x626F60CE’ as the type and use either ‘0x00000000’ or ‘0x00000001’ as the group in the case of stuff made for the base game. 

The instance must be copied from the CASP, this is unique for every single CC unless it was cloned. 

So  once you have your image just drag it and drop it inside s3pe and modify the attributes.

With this you can create your own thumbnail for garments, shoes, accessories and hair in those occasions when it wasn’t included (this isn’t “bad” per se, unless the game generates a wrong thumbnail making the CC undistinguishable from the rest).