From the Materials Lab

To access the Pictures Editor from the Materials Lab:

Select an object.
Click on the button next to the selected object. The Materials Lab will come up.
In the Materials Lab, click on one of the columns in the grid to activate a component window:
In the component window, click on the P button in the bottom-left corner. The default moon texture appears in the window:
Click the Pink button at the top of the window, the Pictures Editor comes up.