On a single server [s1] I for any page I create and re-size one image right into a 300x300 in a single folder and [a thumbnail] 130x130 inside a subfolder. s1 is really a light server. I've another server [s2] running exactly the same OS [fedora 15] serving static files [images, css, js] only, via nginx.

Can you really copy the pictures to s2 once they are produced [resized, etc] from s1.?

This only happens maybe 10-20 occasions each day but must be immediate [if at all possible]. I have find out about using php &lifier sftp but I've been not able to understand it.

I suppose my question, more to the stage, is Can anybody recommend a technique that's reliable.?

Appears as if you may need rsync.

You are able to execute that command in whatever way you would like.

Take a look at couple of good examples. http://www.thegeekstuff.com/2010/09/rsync-command-examples/ (not my blog )