If uploading icons, images must be 2x size so that high-density screens can still appreciate the beautiful icons. Icons should also be white (so that you can change ImageColor3 to whatever you want, including black).
For example, for a 24x24 image (toggle_on) from material.io we would download the following.
Go with the 48x48 image in the 2x folder.
Pixels of 100% transparency (0% opacity/alpha) must have white color values.
To fix this in GIMP, do the following. First, create a new layer:
Make sure it is filled with white pixels:
Next, cut out all of the pixels, leaving only transparent white pixels. To do this, press
R (open select tool), click on the image, hit
Ctrl + A (select all), then hit
Ctrl + X (cut). Click and drag on this transparent white layer and make sure it is on the bottom. Then
Right Click +
Make sure to
save color values from transparent pixels when you export the image.