Apps you may already own:
- PowerPoint
- Keynote (Mac only)
Open Source:
- GIMP
- Krita
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.