Previously I had posted on how to reduce the opacity of shapes in Google Drawing. This trick does not work for images however. I like to use images as background images for infographics, bookmarks, and other student projects. If the image is full color it can get in the way of the content in the drawing.
Here is my workaround:
Insert->Image and place it on the canvas.
The slider on the right side adjusts opacity. Reduce the opacity to the desired level.