Great UI, and a nicer setup than most DMG apps I've tried, with the exception of icon placement! There's no snapping, no moving with the arrow keys, and when trying to move 1 or 2 pixels with the mouse, the icon will either jump back to its original location, or jump 2 pixels further away. This makes it impossible to align icons, or place a single icon in a specific location. Adding to the difficulty, the highlighted style used in DMG Canvas does not match the OS X Finder style, so aligning elements using the highlighted title region will actually NOT line up with background elements once the final DMG is opened in Finder.
If snapping and precise placement controls can be implemented (nudging with the arrow keys would be ideal too), this will probably be the easiest and nicest solution available.