That’s probably because the thumbnail image you’re uploading is smaller than the default resolution of Roblox thumbnails. It’s upscaling the image to the proper resolution, and that’s what’s making it blurry. If you’re able, try remaking the thumbnail with a higher-resolution render.
I think Roblox only allows low quality pictures on their websites. I’m also looking for this issue myself but can’t find any proper reason. Whatever I do it’s blurry. If I found out the solution I’ll tell you.
You can upload multiple pieces at a resolution of 1024x1024 then place them side-by-side. If an uploaded image is larger than 1024 in any dimension it gets scaled down. @Tomarty said this here:
Scaling the image to the size you want would mess up the pixels, the image you posted on the post is 400x225 which is why Roblox is scaling it to 1280x720, which is causing the blur issue you’re facing.
Just look at this pictures you may understand a little bit:
When you upload it may look blurry, but cutting them and placing them side by side will look less blurry.