[Help] PNG + transparency + PSP 4.12

Hello,
The red mist has descended & I need your help JGO. I have a version of Paint Shop Pro (that might be older than the internet itself) and I’m struggling to find a way to set some of the tiles in my sprite-sheet to be transparent.

I was thinking that maybe it might not take too long to simply load one sprite, blat it to be completely transparent (via a one-off java method call) and then save it as it’s own image? Then open it, and copy/paste it back onto the original sprite sheet? …or would that not work?

Suggestions?

ps: I don’t really want to download yet another paint program and then waste months learning it.