And, dearest anon, here is your request.
You’re going to need GIMP because that’s really the only way I know how to do it (and because I’m too lazy to actually figure it out on Photoshop since GIMP is easier)
So you open your picture on GIMP (obviously)
Once you have whichever picture you want to use (I recommend solid white backgrounds, because black backgrounds don’t work if you have other dark colors in the picture) you’re going to want to click the “Colors” tab thing on the top
And then look for “Color to Alpha…” and click that. (I can’t get a screenshot of that ugh)
Once you’ve clicked that, this little fella should appear:
…showing a display of your picture with a transparent background, since the color white (#fff) is selected in that little box in between the words From: and to alpha
You can use that box to select the color of you background. If it’s not white, I recommend you find the # of your color and copy-paste it there.
Click OK and viola~
To save, click File » Export… and then export is as either .gif or .png because I’m certain .jpg or .jpeg can’t handle transparency.
And you’re all set! All questions to the ask box!