The jpeg kind of ruins the comic. And the only reason you could have not to be able to save it under a PNG is if you have an older computer, if not then that's just weird.
But if that's the case then you can just save it as BMP and upload it to Imageshack.us or one of the other imagehosting sites I know of. But I know Imageshack accepts BMPs.
Btw, cool idea for a comic, keep it up!