I'd avoid using PowerPoint personally. I've never seen a good looking thumbnail to come out of Microsoft software. Much better using the TubeBuddy thumbnail generator.
I'll also put a recommendation for photopea. https://www.photopea.com/ Roberto Blake has recommended it, and on desktops, it's a very powerful free alternative to Photoshop all online based. It can get a bit laggy, but it can do a lot typographically that can be limited in some other apps.
I find GIMP has too much of a learning curve for me, and photopea can do similar, with a lot less steps needed.
I typically make most of my thumbnails on my phone with the PicsArt app. It's really good for photo editing. Here's one of the thumbnail examples for one of my gaming videos.
It's available for both Android and iOS, so if you ever need a quick and simple solution that's mobile friendly, I'd also use that.