Yup this is what's happening. Adding background: white; to the image fixes this problem.
This is how we fixed the same problem on mobile (compare https://en.wikipedia.org/wiki/Triglyceride#/media/File:Fat_triglyceride_shorthand_formula.PNG with https://en.m.wikipedia.org/wiki/Triglyceride#/media/File:Fat_triglyceride_shorthand_formula.PNG)
@Tgr after adding the self-hosted checker.png manually to the element using background: url( checker.png );, it works as it should be:
I will test that also out on my local vagrant setup later and see if this also happens when the file extension is uppercase or lowercase.
Edit: Maybe we should apply the background-image no matter what extension the image file has?
@Phantom42 Feel free to claim this task, I am currently too busy doing school, to work onto that.
I know that the solution for this is to find the code section, where the css class for the image file extension is being appended. The extension have to be lower case in order to work properly. :)