Fortunately, I was able to find the answer. If the page with the image to investigate picture element, it is possible to see a link like:
.../owncloud/index.php/apps/files_sharing/ajax/publicpreview.php?x=123&y=123&a=true&file=undefinedmiramarmap.jpg&t=0c0d876ff5edcc9dab0af86efdf50170&scalingup=0
So, this is the link where you can open the image in the browser. The main thing, right ask get the x and y parameters (they can be changed depending on what resolution you need). Option scalingup can be completely removed