I've tried to save sprites on https://dsipaint.com, http://dsipaint.com, and http://www.dsipaint.com, and I cannot save sprites, via DSi. But, the member "britch" is able to save sprites, it seems.
Unfortunately, the Nintendo DSi has very limited RAM. After creating DSiPaint Spriter, this became very apparent. As art gets more complex, the browser does some kind of caching with the canvas, which can exhaust the RAM and cause only part of the art to save. I've tried over the years to resolve this but don't see a way to do so. It has happened with my own art after spending hours creating something, so I know how terrible it can be.
I tried taking up most of the space on the canvas, using multiple colors, using the copy/paste system, different brush sizes, and the erase tool, for about 5 minutes. Still unable to save it.