Just in case this helps someone else:
I was trying to change the icon of an alias (to a .jar file) to a 256x256 PNG, and dragging the image over the icon in the Get Info window didn't work, and neither did copying + pasting the image after highlighting the icon. At least, it did not seem to have worked, but experimentally I tried clicking the "Locked" checkbox in the General section of the Get Info screen (after trying the above steps) and voila! the icon changed. Unchecking the "Locked" checkbox returned the icon to its generic one.
Then I noticed, while the "Locked" checkbox was unchecked (unlocked) I changed the name of the file, and again, voila!, the icon changed, and stayed changed, to the desired one.
I've noticed several icon refresh issues similar to this in Mavericks.
So for someone else experiencing this (at least on Mavericks 10.9.4):
1. Follow the steps above given by Schnapple (drag or copy + paste the desired icon into the Get Info window).
2. If the image doesn't change, try checking the "Locked" checkbox
3. Alternatively, try changing the name of the file.
4. Essentially, after applying the icon change, if you don't see it change, try altering another property of the file.