Hi Carlos, welcome to the forum!
For an image popup on a polygonal hotspot, you can choose Type: URL and specify the URL to the image in the Link Target URL in the properties.
You can use an online URL (https://...) or use a path relative to the output folder of your tour. For example:
media/MyImage.png
Note that for polygonal hotspots, a target image won't be automatically included in the tour.
So you have to make sure the MyImage.png is placed in the media folder of your output manually.
Alternatively you can add the image to the Assets in Output > Advanced
Then you can use the URL: assets/MyImage.png in Link Target URL of the polygonal hotspot.
Currently the image will popup with it's center on the first point/vertice of the polygonal hotspot.
This will be improved in an update so the image will appear in the center of the polygonal hotspot.