So, I was trying to get rid of the obvious flaw in the following picture, and I found a pretty easy way to do it with GIMP.
Now, I’m no expert at GIMP or anything, but all I did was select the region in question using the lasso, like so:
Then, I used the Colors -> Components -> Channel Mixer tool to bend it to my will (make sure you grow your selection to encompass the whole flare using the Select -> Grow tool):
Notice how in the preview window the blemish isn’t visible? That’s what you want. Now, hit OK.
So, now all we have is a little bit of a color/contrast issue. Let’s fix that up with the Colors -> Color Balance tool:
Now, if you look closely, you can still see just a tiny bit of brightness, but that’s alright with me – it’s much better than the blue crap. Here’s the end result:
And there you have it: how to remove those annoying lens flares with GIMP.