Increasing memory usage is normal. You are, after all, loading more data each time - the HTML from your AJAX response, as well as the images that are being displayed. Unless you're using Adobe Pagemill-generated HTML, that's only going to be a few hundreds of bytes of HTML/text. It's the images that will suck up the most space. Everything get stuffed into the browser's cache.
Since you're not doing anything fancy with the DOM (building sub-trees and whatnot) directly, just replacing a chunk of HTML repetitively, eventually the browser will do a cleanup and chuck some of the unused/old/stale image data from memory/cache and reclaim some of that memory.
Now, if you were doing some highly complex DOM manipulations and generating lots of new nodes on the fly, and were leaking some nodes here and there, THEN you'd have a memory problem, as those leaked nodes will eventually bury the browser.
But, just increasing memory usage by loading images is nothing to worry about, it's just like a normal extended surfing session, except you're just loading some new pictures.