A read-only disk image file are configured through the file's properties, which allows you to lock the file to prevent changes from being made to the file. In order to convert the file from read-only to writable you must disable the lock feature, and configure the user settings to permit others to make changes to the file. Once changed, any user who has read and write privileges can edit the DMG file, as well as lock the file.
Hold "Control" on your Apple keyboard and click the DMG file.
Release "Control" and click "Get Info" in the pop-up shortcut menu.
Click the checked box next to "Locked" to disable the file's read-only configuration.
Click the privilege next to the user account's name at the bottom of the Get Info window, and click "Read and Write."
Click the red button in the upper-left corner of the Get Info window to save and close the DMG file's properties.