curious as to why you went this route instead of just placing the html in the footer section of a theme?
i put this in the Footer section of a blank theme and it worked fine.
EDIT CSS/HTML Footer:
<center><a href="https://www.wur.nl/en.htm" target="_blank">
<img src="https://cdck-file-uploads-europe1.s3.dualstack.eu-west-1.amazonaws.com/standard21/uploads/fspm/original/1X/f2f2e6d11a09f6ceadecf437c48b24c8f44ea76d.png" alt="Footer Image" width=300>
</a>
<a href="https://www.uni-goettingen.de/en/1.html" target="_blank">
<img src="https://cdck-file-uploads-europe1.s3.dualstack.eu-west-1.amazonaws.com/standard21/uploads/fspm/original/1X/0da449a3a30e30eaa4467e96189b7bd4e0a28a64.png" alt="Footer Image" width=300>
</a>
<a href="https://algorithmicbotany.org/" target="_blank">
<img src="https://cdck-file-uploads-europe1.s3.dualstack.eu-west-1.amazonaws.com/standard21/uploads/fspm/original/1X/27ca1fc832ecd29153d2ff150011e0dc056d4f93.png" alt="Footer Image" width=300>
</a></center>