Below are instructions for changing the opacity of a background image in a Google Presentation. There isn't a really good easy way to do this but the steps below will work.
1. Open a new presentation.
2. Add a background image to the presentation.
3. Create a new rectangle shape that covers the background image.
4. Select the rectangle and then click on the Fill Icon below:
5. Make sure to fill the rectangle with a grey color.
6. Click on "Custom..."
7. Slide the triangles on the furthest right bar closer to the middle and that changes the opacity on the rectangle that you drew.
8. Below is an example of the box I drew halfway over the background.