It depends on if the iMac was configured with an SSD, Fusion drive, or HDD. If it was configured with an SSD or Fusion drive, then there is a candy-bar style PCIe SSD on the back of your logic board. If the iMac was configured to have an HDD, then there is only a 3.5" HDD on the left side of your logic board.
Also, in OS X, you can open the System Information application and see if there is any entry for 'Apple SSD'.
You can certainly replace this HDD with a standard 2.5" SATA SSD, but I'd recommend a 3.5" bracket so that it doesn't lay in the housing free-floating.
Hope this helps,