thank you. I found it.You'll need a plugin to change the screen size (like 1024x786) (which is what I assume you're really talking about, because changing the resolution (a number like 300dpi/ppi) would require exchanging all pictures, and those two (different) things have always been confused with each other).
There are several plugins in the base plugin pack, and a few more linked in the plugin list that could do this.