You can adjust gamma of individual images and your background.
Gamma and other filters and adjustment options can be found in the right contextual menu. Select any item to access the right contextual menu for that item. Don't select any item to find the right contextual menu for editing your background image.
Click the gamma toggle to open the full gamma menu. Adjust the intensity of red, blue and green colors in your chosen image. Use the sliders or type in a number from 0-100 to fine tune the gamma.