Issues with Dynamic Background Image

Hello everyone,
I am currently trying out the JS for the dynamic change of background images and I am currently experiencing an issue with how the background shows up as it never stretches to the edges - so my question was if I am actually missing some steps or if there are some sizes that I should stick to in order to use this function.

Here is a dummy LP just to make you understand the issue:

Hope to hear from you soon & thanks in advance for the help!