@DannyD
I understand, that you are using Tier 1 Basic, but to implement support for compressed textures, somebody would have to implement it into Tier 2 C++ and than it could be possible, to become a next feature also in Tier 1 Basic.
and for the other way around:
XnConvert, IrfanView, they can do batch conversion to JPEG or PNG. And you could use some optimiser like TinyPNG or RiOT or what, to create smaller files with JPEG or PNG, but the decompression would take time and uses a lot of RAM, if you have a lot going on.
If you need some more infos, I tried to combine these here
https://forum.thegamecreators.com/thread/224558
I had the same problem with my own "viewer". I wanted to create bigger thumbnails and put all assets in one new data base.