I think the large picture of the woman in the bikini might be a little too revealing. You might go with something tamer. I imagine it would make the target audience more comfortable.
Yeah, the picture is super off-putting; I liked the app concept but any interest I had disappeared when I saw that login screen. It looks like something more suited for Tinder than a health tracking app, and frankly, I'm sick of seeing skimpily clad women as the metric for idealness. It's demeaning and demotivating as hell if you can't fit into that mold. Not to mention that some titillating picture of a woman's ass isn't in any way appropriate for a general interest app in the first place.
I think it's fine. Although it should show either a fit man or woman, depending on what sex you choose. Something to inspire you with the will the continue on the iron path.