In Firefox a Reload seems to reload everything while on IE the Refresh reloads just the HTML part of the current page. In IE you need to press Ctrl-F5 for a complete reload.
Why this difference and is it somewhere an article explaining this difference more thoroughly?
Firefox employs a cache like all modern browsers, so it doesn't load everything on refresh. Different browser engines handle caching somewhat differently, but it mostly depends on the headers you are sending.
You should check the w3 page on caching in http and also this caching tutorial for more details on how web caches work. Also this article on the differences between firefox and IE caching should be relevant.
Ctrl-F5 is reload without cache, in IE.
F5 should reload images too, as long as the browser detects that they're changed.