For me, each icon's density specific version goes in the following folders:
/Resources/android/images/res-ldpi/icon.png
/Resources/android/images/res-mdpi/icon.png
/Resources/android/images/res-hdpi/icon.png
/Resources/android/images/res-xhdpi/icon.png
/Resources/android/images/res-xxhdpi/icon.png
/Resources/android/images/res-xxxhdpi/icon.png
/Resources/iphone/images/icon.png
/Resources/iphone/images/icon@2x.png
And then I reference it in .js:
var myImage = Ti.UI.createImage({
image: '/images/icon.png',
});