This was tricky…
I kept getting:
iPhone/iPod Touch: Icon.png: icon dimensions (0 x 0) don’t meet the size requirements. The icon file must be 57×57 pixels, in .png format (-19014)
I found the fix here:
MacOSX 10.7.3 Update Breaks iOS App Validation.
Thanks Manbolo!
Basically, get the latest version of Application Loader, run it and restart Xcode and try again after a clean build.