Second picture disappears after merging layers

Hi All,

I'm trying to get 2 separate images together on one layer. I thought I could accomplish this by first creating 2 separate layers and then merging them into 1, but the second picture doesn't seem to make the transfer. When I click on the layer that should have both, I only see the original picture. 

In the below screenshot, I want the picture from the "3VBox" layer to be merged into the "4/3V" layer, so after selecting "3VBox" from the options on the left (bringing it to the middle) , I would click on "4/3V" on the right box and then select "OK". Once I do, the "3VBox" layer does disappear, as expected, but its picture is nowhere to be found on the "4/3V" layer. Am I doing something wrong in the below screenshot, or do I just not understand how merging works? Thanks!!