oops, sorry, fixed it now!
That was my fault, I rushed it late last night, there was a skoolboy error in the first line too, so also fixed that - try it again now 
No guarantees it’ll work to pull in background images in CSS though - but it might.
Probably worth noting, while it should work when the exe has been compiled, there can be issues if the page calls too many images at once, say for example you have 100 small images on a page all attempting to display upon page load, exeoutput occasionally throws a system error - can’t remember which error it was, but it was enough for me to have to limit the amount of images displayed on a page to ensure stability.
You can experiment with the different headers as I don’t think all are required for the technique to work, those you see there are just the result of me meddling around attempting to fix the above issue that I had to give up on in the end - but it’s the exact code I currently use, so it should work fine for now - just be mindful of trying to call too many images on a page at once.
Maybe the dev might have an idea about that problem? (I did screenshot the error somewhere, so if I can find it I’ll post it - I’d planned to do that ages ago, just been flat out with other work of late)
EDIT: It was a list index out of bounds error, occasionally followed by a php-cgi.exe has stopped working…:
As is often the case with errors, it was pretty random, sometimes I’d not see one for a while, then have several in a few hours use, but eventually tracked it down to the images. Once the images had been cached in the browser it was not a problem regardless of how many were on the page, subsequent page refreshes with the same images using the above technique worked fine - but loading a page that called 100 brand new images via php would occasionally cause the crash. The issue only started after upgrading to exeout 2018 as I recall - and still seems to affect the current version.