Hello, what do you mean with custom widget? Picture?
Every widget can have custom foreground and background image. Go to widget properties and press orange plus near image design on top. Images are automatically downscaled to 192×192 if are larger.
If you need image with higher quality you must put image manually to game profile folder located in magicbox/games/data
Larger images are good if you play with multiple layers and you want use big image like a menu (f.e.)