How in the world do I make my thumbnails not grainy

Needless to say, it’s mostly a technical problem, but everything I make has to be super small when publishing things, and super small PNGs make the picture grainy, How can I stop this problem from happening, because it’s stopping me from publishing my maps.
I thought it might be because of my computer, but I’m having second thoughts, because I’ve seen this sometimes happen to others on Discovery too.

It doesn’t have to work, as long as it appears, just scroll down and they’ll take it.

It has a designated size limit which prevents my picture from getting through, no matter the size, which is kinda annoying having a small amount of space to work with.

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