Select the inside of the frame, click on the new layer mask button at the bottom of layers panel. If it is backwards, hit cmd or cntrl + i with the layer mask selected and it will invert your selection. You can also double click on the layer mask and a properties panel will pop up - it has an invert button.
Thank you I will following your instructions to invert the selection
Consider an option where a mask is not needed at all. I imagine you have a template with transparent cells such as the one shown here. If that is the case:
Use the Magic Wand set to Contiguous in the Options bar to Select a cell. Marching ants will appear.
Choose an image, such as the one on the left. Select > All followed by Edit > Copy
Return to the template. Choose Edit > Paste Special > Paste Into. The result will be Figure 2
To resize the the image to fit, Edit > Transform > Scale. Hold down the Shift key to maintain proportion.