I'm looking for a way to make a click in a default gallery not open the image file but replace the first image in the post with that file.
Imagine a post that starts with an image and below that, say, 3 or 4 thumbnails from a gallery. All done with out-of-the-box WP/twentyeleven features. The idea is that if you click one of those thumbnails the first image in the post gets replaced with the enlarged version of the thumb you clicked (obviously the original first image needs to be among those thumbnails as well to be able to click back to that one after changes). This needs to work in seperate post pages but also in post showcases like the homepage or category pages.
But I'd rather stay clear of the WP core. Does anyone have suggestions on how to realize this? Are there plugins that do this that I haven't found? Could I use a filter? This is my second day on using WP so maybe I'm overlooking something obvious. But any help would be appreciated!
FYI: I'd like to use this method to slim down the scroll length and increase load time of pages while still giving visitors the ability to quickly view several images of a post in a larger size. Without having to use self-themed or flashy galleries in every post.