You can certainly come to a profit sharing agreement and there is some good feedback from other experts listed here about how to structure that to protect yourself. However, most technology startups and mobile apps need to be reinvesting their profits back into the company, so even though money may start to come in, it doesn't mean you want to spend it on paying back the designer you worked with. When I started my company, I gave a designer 1% stock to design my logo, business cards and the four main pages of my website. This was a great deal for me and would have been an incredible deal for them if we sold for a big amount in the end. But most importantly, it didn't use up any of my precious cash in the early stages of my business.

You may not have a lot of money coming in for awhile. I would try not to spend any of it at all if you can find someone willing to work for equity instead.