cool! have you done anything with player custom sprites or sprite sheets?
I thought about maybe letting the player upload a custom sprite as one of the obstacles/pick-able bonus, but not sure I will, I did do it before though mostly for drawing apps.
In Swipey I am taking a scale-to-fit screenshot of the layout for the level preview and sending it with the game when you post it, that sort of the same.
My only advice is to use something like canvas or paster to show and store imported/runtime generated images, because firstly, you can have as many instances as you want, each with a different image (cached separately), if you use a sprite object you will need to add a bunch of empty frames up front and use different frames for different images, it would be better if you could add frames at runtime, but you can't
Secondly canvas/paster is also better because you can redraw the image at a lower resolution and crop it if necessary before saving it, users don't really know what they're doing, they can import a 22 megapixel image, it might load up on the sprite ok, but getting this extremely large dataurl and storing it wouldn't be easy and would probably crash the user's browsers.
If you need sprite sheeting for animations, I still recommend getting each frame with canvas/paster rescale it to a maximum reasonable size and then import each frame to a pre-made sprite objects with enough frames to cover your basics.