I'm writing an app that displays a number of different-colored, crystal-looking marbles. At times, I need some of the images to appear slightly blurred. I would like to make sort-of a hazy looking circular cloud-ish image that I can display over the tops of the marble images so they appear a little blurry. It's too slow to actually blur the images in real time, and there are too many of images to open and blur each file manually beforehand. Hence, the strategy/theory that I might be able to get close with a single, semi-transparent overlay file. But that's as far as I've gotten (idea stage). Any tips or pointers on making something like that in GIMP? Thanks for any help!

I should have added: I also need the edges to gracefully fade to completely transparent so as to blend with the background surrounding the sphere.

Possible strategy: Take one of the marble images, gray scale it, blur the grayed image, make it semi-transparent, and then display that over the colored marbles. Can't figure out how to replace everything in an image with a slightly more transparent version of the same color, and opened just that question in a separate thread. https://www.gimpusers.com/forums/gimp-user/21863-ht-make-an-object-semi-transparent