I was able to fix the issue.
As I used Greasemonkey is the past (2010 something) I installed this add-on again for this script. However, as you were refering to Tampermonkey (I was thinking this was like a chrome-addon for the same purpose) I eventually looked it up and it worked out-of-the box. (I never used userscripts except if I really had to)
So for the people having the same issue: It does not work with greasemonkey.
As I used Greasemonkey is the past (2010 something) I installed this add-on again for this script. However, as you were refering to Tampermonkey (I was thinking this was like a chrome-addon for the same purpose) I eventually looked it up and it worked out-of-the box. (I never used userscripts except if I really had to)
So for the people having the same issue: It does not work with greasemonkey.
Re: [Script] WME Image Overlays (1.0)