The max image size you can upload is 1024x1024 without downscaling.
If you upload something bigger Roblox will just shrink it down to 1024x1024.
When uploading images there should be a warning letting you know that the image will be downscaled.
This isn’t a big issue but would be nice to have and thought it was worth mentioning after having to upload multiple of the same image because I didn’t know about the size limit, and was wondering why my image was coming out blurry.