Can I customize the resolution window that appears at the start of my executable?

Can I customize the resolution window that appears at the start of my executable? Is it possible to put a graphic at the top of the window, change the layout or change the available resolutions?

The banner graphic at the top of the resolution window can be changed by dragging a texture to the "Resolution Dialog Banner" field in the Player settings. Standalone builds targeted to OS X include several ".nib" files, which are editable with Interface Builder(wikipedia, user guide), a part of XCode, Apple's IDE. The resolution dialog can be found by secondary-clicking an OS X Unity .app file and selecting "Show Package Contents." In the window that appears, select Contents/Resources/ScreenSelector.nib (if you have XCode installed, it will appear as a metal plate with bolts in the corners). Advanced editing techniques in Interface Builder are out of the purview of this site, but moving items in the layout around should be simple enough. Be aware that replacing a build will replace this file, so keep a copy in a safe place.

Another thing you could consider: You could disable the resolution Window altogether and create your own custom GUI startup screen. I'm not using this (ugly, IMHO) window at all, so I'm not sure if there are settings only available in there - but you can certainly do your own resolution and quality settings GUI and then set those in your game code which works just fine (Traces of Illumination does that, even though I don't do this in a "resolution window" but instead have a "configuration tab" in the game settings where the user can select screen resolution and a couple of sound and quality settings; the actual quality settings I change from code dynamically depending on FPS; but that can be disabled and then you can manually set a quality setting - which also helps me a lot during debugging specific quality levels ;-) ).