I’ve tried to do this with the opacity setting for the image and the opacity setting for the shape the image is located in and then using the graded fill settings. Neither seem to work because to make the image transparent it seems to be necessary to also make it literally fade away. It looks to me that I need to use a different photo editor to do this. Can anyone confirm?
The best way that I know of to accomplish this in OmniGraffle would be placing second rectangle (or other shape possibly) on top of the image. You can then use a gradient fill on this shape that fades from transparent to black. Here’s a quick before and after example of what this can look like:
I’ve tried that on a photo but the “transparent” part of the gradient fill affected the photo underneath (it lightened it slightly). How are you making the transparent part completely not change the image beneath? Or how are you choosing one part of the gradient fill to be totally transparent?