Here are my picks for the top five most iconic covers featuring Jimmy Olsen (with iconic being determined by what covers are most established and most recognized when it comes to Jimmy Olsen, with a preference towards covers that are homaged a lot). This is not a BEST cover list and due to the very nature of icons, very few (if any) recent covers will be on the list. A notable exception to the rules is that I don't count covers from a character's first appearance (which isn't applicable to all characters, of course, just those who appeared on the cover of the comic they debuted in), as those tend to be automatically iconic so they're boring. Here's a list of all characters featured so far.
5. Artists: Curt Swan and George Klein
This cover nicely captures the hilarious absurdity of a typical Jimmy Olsen story with one of the more striking covers of the era.
4. Artist: Neal Adams (with some Murphy Anderson corrections)
Here's another one, coupled with a striking (no pun intended, honest!) Neal Adams cover that stuck in folks minds well after the issue was gone.
3. Artists: Curt Swan and Stan Kaye
Swan's cover for the first issue of Jimmy's solo comic is about as pitch perfect in capturing the essence of Jimmy Olsen as you could get - if the other two covers ahead of it weren't so famous, this one would definitely be #1, as it's the clear #1 strictly in terms of "best summation of Jimmy Olsen as a character."
2. Artists: Curt Swan and Stan Kaye
Easily THE most notable transformation Jimmy Olsen ever had (cover-wise, at least). Some points taken off for being an homage of an old science fiction book cover, or else this would be #1.
1. Artists: Jack Kirby and Vince Colletta (Al Plastino Superman face)
Kirby is here!
Who else but Jimmy Olsen could luck into having Jack Kirby use his comic to launch the freakin' Fourth World!?!?!?
Extremely iconic cover and moment, and Jimmy was right there in the middle of it (sticklers for detail will note that Superman's face is drawn by Al Plastino).