Full Background Image Doesn't Display

Hi there. I installed this plugin to use with the “Mantle” theme. I uploaded an image that is 1707 x 2500. The plugin only displays the upper left hand part of the photo instead of the full photo. Even at smaller resolutions (I tried 25% of the picture size), it still only displays the upper left hand portion. Do you have any suggestions for getting the full image to display? Thanks for all of your help!

http://www.openmindednessandotherlies.com/

https://ww.wp.xz.cn/plugins/simple-full-screen-background-image/