Okay Paul. I think to maintain the transparency of a png image, you have to download (save) it to your PC before you do anything else. Just copying the image and pasting it into an application will lose it's transparency.
MS paint is not that great an application to be honest. If you're after just basic stuff, then it's probably okay, but there is much better stuff out there, and free too.
I've had GIMP on my PC for a while and although I don't use it for most of what I do, it is a very capable picture editing program. I'm sure there are other programs too, but I've not used anything else so I can't say.
Going from MS Paint to GIMP will be like being thrown into the deep end of a swimming pool, but it is SO much better. You will have to learn a fair bit to use it effectively, but believe me, you will be rewarded for your time.