Working with art in the internet I end up knowing more about copyright law than I ever wished.
One of the biggest caveats is, while some images have public copyright licenses which may not allow modifications or use in commercial works. It's very easy to get used to use goggle images for your school work and keep the habit when you start working.
The commercial part is specially complicated and to play safe I prefer to err in the side of caution when using stuff on Nerf NOW!! This means paying for my fonts, my softwares and images when appliable. This is specially complicated when working with other people IPs because there is just so much the parody clause can cover and one of the reasons I don't like to go too deep into any IP.
The background in the image is public domain (CC0) thus free to use, modify and publish even in commercial works.
However, in the comic itself, the image would be a stock photo copyrighted image and would not be free to use.
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