Above are basic instructions for covering an entire image with a given color, but transparent overlays can be much more complex. Overlays can also include gradients, and do not have to cover the whole image.
Partial overlay effects, or shape overlays, can be just as effective as a full overlay. A band across the top, bottom, or side of the image can provide a sense of unity just as well as a full overlay can. Text effects on overlays, in your branded font, can further incorporate images and stock assets into your brand story.