Did a little experiment...
Paste some sprites into a Drawing Canvas, save Drawing canvas, get blobURL, then run a little JS to convert the blob URL to base64, put that in a string and load a Sprite, seems to work (the one caveat is that the Drawing Canvas transparent regions get changed to black...
So if you want to save a DC as a string which can be used in JSON then load that image back into a sprite, this is one way...