apache - Cache control headers, cache for long time, but still validate? -

i have url i'd cache (forever per se) but i'd have browser re-validate everytime. don't know if it's possible, either last-modified (see below) or if size different.

i'm trying find out how/what "sets" last-modified response header. did test example "max-age=86400" , have image showing:

cache-control:max-age=86400, public date:fri, 29 jan 2016 04:21:31 gmt expires:fri, 29 jan 2016 04:21:32 gmt last-modified:fri, 29 jan 2016 03:59:25 gmt 

yet browser still caching it, , that's fine expect that, but problem (and question) last-modified part. one, image different size , modified since 03:59:25, i'm expecting browser re-check if file changed or if should serve cached image. if 'stat' file @ terminal modify time not 03:59 anymore, i'm curious if last-modified time because that's when "last-modified" when browser cached it?