There is a missing “filters” parameter on the rbxthumb:// documentation. You can do filters=circular to get a circular image, which can be extremely useful in UIs especially with AvatarHeadshots. It’s quite annoying to have to find this parameter as it doesn’t even have an intuitive name that I can easily remember.
In some situations it is more ideal to have Roblox render the thumbnail as round rather than messing with UI corners. Why add a UI corner when you can simply add a little line of text to get the image you want?