This update adds support for custom spritesheets!
Drag and drop a PNG image into the editor and the "Import Spritesheet" modal will appear. Choose a name for the spritesheet (this is what will appear in the spritesheet picker dropdown in Settings) Specify the tile width and height. This is the size of each tile in the PNG image. They must be aligned on a grid. Specify the scale. The default spritesheet has tiles that are 48x48. If you are uploading a 16x16 spritesheet you can choose 3 and they will be upscaled. For a 24x24 spritesheet 2 would be a good choice. Choose "Includes Active State" if each tile has a corresponding state for active beneath it in the spritesheet. The spritesheet can be any number of rows and columns. If the sprtesheet includes active states then each row will be two tiles tall with the default states on top and the active states below. Here's an example of the default spritesheet. It includes active states. Feel free to use it as a template!
To choose a spritesheet select it from the dropdown in the "Settings" panel:
I hope you enjoy the update! - Daniel