If you use high quality images, it is possible that scrolling through your video collection in MoviePlayerApp is slow or stuttering.
It is also possible that some images (really high quality) don't load at all. Because images are loaded over your network, the larger the files are, the longer it takes to transfer them from your computer to your device. MoviePlayerApp does cache these images on your device, so after scrolling through your videos, things should be considerably smoother, but if your images are too large, it could still cause complications.
For best results, make the backgrounds the same size (or a little less) as the screen size you are viewing them on. On some devices with an HD resolution it is possible to display images up to 4x that size, but it slows things down unneeded.
For cover images, use a maximum height of about 70% of the height of the screen (700 for a 1080 screen or so), and if that doesn't help, you can turn off "rounded corners" and "mirror" in MoviePlayerApp's settings to speed things up a bit as well.
To resize images quickly and easy, we recommend Image Resizer for Windows (link). After install, a simple rightclick on your images gives you the option to resize your pictures:
After which, you can select "Custom" and enter your own desired sizes:
For the images used in the sample trailers for MoviePlayerApp, we used 1280 x 720 for the background images, and 600 x 400 for the cover images, which should have them look and work great on the majority of different devices.