This fire image:
As you can see, it has a black background, which I can't delete (because it would probably destroy the smooth-looking fire effect). I want to place some fire in a photo.
However, I remember it was possible to take such images and do something in photoshop so that when you place them over a photo, the black is gone and the fire is perfectly blended there. But I can't remember what feature or option was it. Any ideas?
On the Layer window change the mode of the fire layer from "Normal" to either Screen or Lighten and see if thats what you want. That's the easiest way to remove black.
Beyond that you can also go into the Blending Options and adjust the slider at the bottom where it says, "This Layer" and move the black tab on the left side in a bit but the results aren't necessarily that good for something soft like a flame. Hold the Alt key and hit the right side of that black slider and it will separate then you'll get much better results. I've got a video showing this method here: https://www.youtube.com/watch?v=F6IhXVxBSQM
A third option if you're just looking for a flame and not necessarily that flame is to look for a flame brush. I know bittbox has some free flame brushes that might work for you to achieve the desired end result.
In the GIMP, getting rid of the background would be as easy as opening the file, selecting Colors → Color to Alpha... and choosing black (
#000000) as the color to make transparent. Here's the result, saved as a transparent PNG:
(That said, in this case using the Screen blending mode, as Ryan suggests, may well be easier and give a more natural-looking result. In just posted this answer to point out that you can in fact delete the background without destroying the smooth edges.)