I used to use Canva.com. It's super simple to use and you can get pretty far. However, I found that some very useful features are missing from Canva, which is a shame. fx. one cannot add an outline to, say, text or image. You can also not skew or transform freely text if you want to get creative with it. You can only use their, rather limited, font styles. I think it's a shame, because I like the tool, but as I found myself opening up Photoshop more and more in order to do stuff that Canva couldn't do, I just switched entirely to Photoshop. You can do everything in there, as you probably know.
I think it's safe to say, that you will not get anything beating the features og Photoshop forthumbnail creation. Question is whether or other tools, like Canva, fit your needs.
Many people also recommend GIMP for a free option as opposed to Photoshop.
Cheers.