Clic on Mount; then you can eject it from desktop

It is the disk image for the Flash Player installer that you downloaded.

It is not mounted, so it will not show up in the Finder.

With it selected as shown in the second screen shot, click the mount button. Then, eject the disk in the Finder.

With the size of the photos you have quoted, it is impossible to make anything even close to a 4X6 print. You would need a photo file of at least 800X1200 pixels

… it's how to delete the password protected one I created.

I guess the answer depends on what you mean by "erase."

To delete the image,

If mounted, eject the disk, then just trash the dmg file and empty the Trash.

An article for it : macOS Sierra: Compress or uncompress files and folders