Thanks for the swift response! Since I had more than 20 images even before the button was grayed out, I figured there might be a different cause.
After removing sections in the document one-by-one, I narrowed down the problem to one table. Whilst I don't know what the ultimate cause of the problem was, it was something to do with that one table. After removing it, the button became active again. So, I recreated the table, and even after inserting all images again, the button is still active 