Arr, GIMP is your best bet. Paint sadly doesn't work for heightmaps, although I'm not sure why, personally.
I think your parents are right for being cautious about what you download on the internet, but I assure you that GIMP won't harm your computer. It's used and downloaded by thousands of people, and the program is consistently updated by a trusted site/organization. And, unlike photoshop, GIMP is free to use.
Not to mention, it's fairly easy to use once you get the hang of it. It's very easy to get any art (as well as heightmaps) looking professional, so it's worth the download.
Just talk to your parents about it, but I understand if they're still cautious.
However, if you aren't comfy with downloading GIMP, you could PM your heightmap to me (if you've already made it in Paint), I could save it in GIMP and do anything else to it that you'd like (since I make maps all the time, it's no problem), and then I could send it back for you to use. Since it will have been saved/edited in GIMP, the version I send back to you should work for you.
I figure that would be a way to do it, too. xD
Either way, I hope ya get things working~
