Gimp - one of the greatest professional apps ever to be found out there... professional image manipulation software; and, of course, open source.
Too bad there are a lot of idiots in this world - starting with myself - who don't have the slightest clue on how to use it. I mean, I always loved photography and as a kid I used to play with my grandfather's "borrowed" old
Zenit camera, but since photographic films were not cheap and I had no means of editing the photos, I quit. Since then, I've been planning... and planning to get a camera of my own, but since I don't want anything below a
SLR - and a decent one is around 1,000 EUR - it'll have to wait. But that's another story...
Getting back to Gimp. My concrete problem was how to get two images of exact size (after adjusting their size such that they "fitted" the dimensions) overlap with an amount of transparency. I'm aware this must be an extremely easy task for anyone just a little bit more photo editing savvy than myself, but anyways - I had no clue what to do so after googling and reading a little bit, I found out.
I'm posting here the "short version", suitable for my concrete issue. (Such that I'll know next time where exactly to go to for instructions.)