enabling the next button only when all interaction on a slide is complete without using variables

Disable next button when timeline on slide begins.

Enable Next button when each picture state is Visited. It requires each picture have a Visited state (which can look like Normal, or something distinct). 

This method used to be flakey if it referenced more than about five objects, but I think that may have been improved. If you only let them click on pictures on this slide, and don’t allow them to visit another slide, this will probably work for you.

If there is a possibility that they may revisit the slide, then the slide needs to reset to initial state for this to work.

This method usually will work, but I can tell you that using variables is a far preferred method, because it is much more reliable, and generally less work to implement.