The problem is caused by Simple Image Gallery Pro, indeed this extension works using background images and dummy 'transparent.gif' images in a way that is not compatible with the Google AMP format.
To make Simple Image Gallery compatible with AMP pages, you will need to override the Simple Image Gallery template a bit. The <img tag's src attribute or data-src attribute, need to point to the real image link instead of the transparent.gif file. You can either do this only on AMP pages by detecting an AMP page in PHP or hide the added image src using CSS on the desktop template.
As an alternative you can consider to migrate to a different gallery component not having this kind of issue with AMP, a great replacement of Simple Image Gallery is: https://joomla-extensions.kubik-rubik.de/sige-simple-image-gallery-extended