QGIS Web View Widget does not load images properly

I have developed a custom slider (html5 + js + css) to be able to add multiple images and files to any inventory item, not only to add it in QGIS but in any Web GIS, corporate webs, etc.

The slider is loaded as a Web Widget (url) that takes the item id as a parameter.

Most of the times when opening the form the image is loaded like this:

Sometimes when opening the form in QGIS the second time it is then displayed ok but some other times it keeps beeing displayed always like this…

Any clues on why is this happening?

If I open the url in any web browser it is always displayed ok, the problem is within QGIS.