I have actually found an answer from back on 2016, in case anyone finds this post. I will repost the answer here, very easy fix in 2017, Miguipigui wrote the following: "I had the exact same issue. I'm extremely new to MOHO and basically any drawing/animation, so I wasn't sure if I was doing so...

Hello, all. I have just upgraded from 13 to 13.5. I import moho files and png files they appear as white rectangles. Also interestingly, when ctrl R the render shows the art, no problem. This could also be in the 'how do I...' category of this thread as it is possibly something I am doing. Thank you