How to merge back the encrypted folder back to main drive
You've created a disk image, which is basically just a file. You haven't partitioned your Macintosh disk, as can be seen if you select Macintosh in Disk Utility.

To remove the disk image, unmount the disk image using the Eject button next to Disk Image, then delete the file using Finder. This will reclaim the space on your Macintosh drive in the same way deleting any other file will.