Ok, searching on the HTML Reference and thanks to this advice I solved it this way (probably not the most elegant, but it works)
At the end of each form I added this code, one line of this same code for each user that works with my OpenMRS implementation.
<includeIf velocityTest="$session.encounter.creator == 'username' "><p><img src="URL-of-the-image" style="width: 200px"/></p></includeIf>
What this code does is verify the creator of that form and add an image corresponding to each username, in this case the signature.
I leave it here published in case anyone is interested in my non-elegant solution.