This is what it looks like on my phone. (I have a folder for my "apps" obsession)
Tap on app pic.
This is the first screen. Either scroll thru these at your leisure or tap on the 3 lines. (where I circled)

Click on the 'Christmas' category.
Now you can browse these frames.
I check marked the one I chose.
Here, click on that symbol to indicate your gallery.
The gallery part of my phone. My 'Jon' folder. Oooo sexy!
I chose a pic.
Notice the edges of the pic are visible within the frame. You need to adjust size of pic to fully fit into frame.
The plus / minus symbols are to add or subtract the photos.
When you adjust the pics, you can see dotted lines all around the pic. That is original edges of pic. So, so long as you like the pic hit the save symbol at top right.
It will save to an automatically made folder called 'photomotager'. It's very fast.
Just an example of the tilt option. Here you can see the dotted lines. You will need this option for frames that are at an angle.
So here is an example of a foggy one. You and see the lines/edges of pic.
(ugh, can't get words to the side of pic)
But here is a slight enlargement. You can still see the lines.
Enlarged more. Now you cannot see the lines.
IF YOU HAVE ANY QUESTIONS LET ME KNOW. I AM HERE FOR YOU. <3 <3