You can use
BLPNG Converter to change to .blp's. Read the page, it tells you what it can convert to .blp's. Gimp2 can export them all, so it's a "win-win" situation. You can use
Blpc to convert various formats, also, but I linked it because it is a .blp viewer. You can get
BLPView if you want to view .blp's natively on your machine. It's a shell extension, so it will provide thumbnail views on a Window's machine. I have a Window's 10 machine, and all of them work for me.