Hey there - nice plugin!
However, there are things I’d like to seen changed 
First of all, your used images suffers from black outline pixels, as talked about in Quenty’s article
Provided below are some spritesheets without this issue - feel free to use them.
(Edit: right-click the image and save it, instead of copying it to your clipboard - saving the actual file keeps the original data, so you won’t get black transparent pixels that way)
You also seem to generate a bunch of ImageLabels - why not use 9-slicing?
Here’s a tutorial on how that works: How to use SliceCenter (Roblox's 9-Slice GUI Property)
Besides that, it simplifies your plugin a whole lot by using 9-slicing - you only need to upload 1 image for each radius size, and you only need 1 ImageLabel to get it all done, making it easier for you to add to your plugin and for people who use your plugin to adjust the generated ImageLabel later on. Simply set the SliceCenter property to Rect.new(d / 2 - 0.5, d / 2 - 0.5, d / 2 + 0.5, d / 2 + 0.5) for the above uploaded spritesheets and it should work like a charm with 9-slicing (where d is assumed to be the diameter of the circle / width or height of image)
As for suggestions to the plugin:
- With enough small increments, you can change your current list-selection for the size into a numeric spinner or a slider, making the size selection step feel smoother.
- Currently the generated ImageLabels are sized outside of the boundary of the GuiObject used when roundifying it - you should definitely try add an option (or make it default behavior) to keep the rounded borders inside of the frame, as roundifying a container element currently makes its size not represent the actual size of the container element. (Although I do see it might be an issue for ImageLabels/Buttons with an image set)