[Feature Request] Provide image thumbnails as filepaths/URLs

[dmarx]

it looks like you do some clever stuff under the hood to resize images and partition them into spritesheets. my images are currently on disk in PNG format (512x512): since this tool is going to do it's own i/o and preprocessing anyway, it would be nice if I could just pass it filenames for it to work with however rather than loading all the images into memory before I can pass the thumbnails into the widget.