Once you are happy with your collage, you can export it to one of the standard JPEG, PNG, or TIFF image file formats.
Use either Export As, or Export JPEG, or Export PNG, or Export TIFF from the File menu. You can also use the Export Image File button in the top toolbar of the application.
Select JPEG, PNG, or TIFF from File Format and save.
Use File > Add to Photos/iPhoto menu.
You should export as PNG if you are using a transparent background.