It all depends, and it could get confusing depending on what's actually in the disk image.
Some disk images contain .pkg or .mpkg installer files, which can be executed (double-clicked) directly from the mounted disk image and will run through an installer program to install the application in the correct place. Sometimes the installer is made by a 3rd-party, like VISE, where you would simply double-click the installer and the installation process will begin -- in this case, it wouldn't have a .pkg or .mpkg extension.
Other times, the application itself (usually with a .app extension) resides on the disk image, whereby you would follow the instructions above -- open the disk image, grab the application in the disk image, then copy (drag) it to your Applications folder.
Yet other times, as bobw said, there could be many, many files in the disk image -- this is rare if the disk image is an installer for an application or is an application itself -- it usually occurs when people make a disk image themselves from a folder containing many files. In this case, it would be up to you to determine where those files go if there isn't a README or some instructions included with the files.