Hi Michael, I'm wanting to do the same thing as you.
I found a solution which might be useful depending on your needs.
1 - make a button block and on the options click file, then upload your image. Click Save.
2 - once you've saved and the image is on the site go back to edit the button, click file, copy the name of the image you uploaded.
3 - make a code block and reference that image that you just uploaded.
Not an ideal fix at all but it is just about possible to get them on the site.
I'd love to see WebP support in the future, it'd be a great feature to have.