fun memory question

I'm running Debian and apache web server.
Its a picture/photo server.
I'm running xosview to watch memory , buffer memory and cache memory.
What I'd love to know is which jpeg pictures are in cache memory ???
Is that even possible ?
Can I somehow monitor the cache ?
I would like to be 100% sure that the last .jpeg I posted is in memory
for the next http user that comes along...

Here's your bonus for helping out...