Blender can be used to render roblox characters and make high quality thumbnails. Blender is a free option that isn’t too hard to learn with many tutorials explaining how to make thumbnails quickly. I recommend using a blender tutorial to make your thumbnail if you want to have 3D roblox characters in it. If you want it to be 2D paint-net, photoshop, and illustrator are your best options.
Also, I believe roblox studio doesnt have any sort of rendering so unless you’re tracing a screen shoot it has nothing for thumbnails.