changing opacity/transparency of an image

[Erin]

I am trying to build an experiment where I incrementally "black out" an 
image displayed on a black background.  I want to be able to set the image 
opacity from 0% to 100% in 2% increments.  I will keep the image opacity 
level constant within a session, but want to be able to change it easily 
between sessions.  I know I could achieve this in Photoshop by putting a 
black background behind my image, adjusting the image's opacity, and 
re-saving the image.  However, I want to be able to draw randomly from a 
very large bank of images for each stage of the experiment, and it would be 
prohibitively impractical to adjust all the images manually in Photoshop 
for each opacity level.

I found a post in this forum on how to fade between two different colors 
(How to fade from slide1 to slide2 (inline?)), however I haven't been able 
to find any information on adjusting  opacity/transparency.  Any help would 
be very appreciated.  Thank you!

Erin

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