I think you’ll probably find that most of that is cached. Linux has a habit of using loads of ram for caching and then giving it back in chunks as needed. Perhaps someone with more of a grasp could explain it to you a little better… but I’m guessing that your RAM is probably not going to cause you and issues.
If you want to test that out though do some benchmarking with your images etc before and after dumping the cached memory… Try this simple little script I use on some of my servers for ejecting it…
#! /bin/bash
clear
echo MEM before cache clear up:
echo
free && sync && echo 3 > /proc/sys/vm/drop_caches
echo
echo MEM after cache clear up:
echo
free
Write the above into a text file with vi/nano/whatever and chmod +x the file then run. Sorry if that’s patronising in any way, I just dont know what level of experience you have.