here are a few photos from my gallery to help you out!!!
i highlighted the outlines and important spots in all three of them, just to show you that you CAN see the knee cap in nearly every photo, and that even if it's hard, the lighting and dimension in the actual photos tells you where to position it so that it makes sense

i messed up the one above a bit but focus mostly on the right knee and how it looks lmao

aalso messed up, the right should have a little horizontal line like in the pic above but these all took 2 seconds lmao

anyways, really hope this helps bb, you're an amazing artist and it's a fckn honour to help you lmao