For the basic instructions, please see the Making Images With Background Pictures section. If your final disk image does not have the icons in the right positions:
Make sure that your configuration has a layout selected.
Check that your layout includes all of the files whose icons you want to place.
When creating the disk image, make sure that the source folder you are dropping onto DropDMG includes all of the files in your layout.
The files in the source folder must have the same names as those in the layout. If DropDMG encounters a file that is not in the layout, it will leave that file’s icon in the default position.
In rare cases, there may be something wrong with the Mac that’s mounting the disk image. You can try doing a safe boot or deleting the Finder preferences file:
/Users/<username>/Library/Preferences/com.apple.finder.plist