Open a new transparent image 400 wide 500 tall.
Open the picture you want to use as the frame and the image you want to frame. The two pictures I used are shown below:
Select the picture you are framing by clicking on the title bar to make it active, then click Edit>Copy, then click on your blank image and click Edit>Paste as New Layer.
Click on the selection tool and select Eclipse, no feather. Make an Oval in the center as shown:
Invert the selection by clicking Selections>Invert to select everything outside of the ellipse as shown below:
Click on the clone brush . and set the setting as shown below:
Right click on the shell picture with the clone brush, then click back on the picture you want to frame. While holding down the left mouse button down, start painting on the inverted selection. The shells will paint inside of the selection. Continue this process until you have shells painted all the way around your picture.
FYI: You can rotate the picture fill on certain pictures to get a really unique look. Cloning is very forgiving. If you make a mistake, just clone over it again.
I added a drop shadow to the oval, and a picture frame. This is my finished product: