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.)
Step 1. Get two images and, after making the suitable adjustments for each of them, be sure they have the same width and height. Have both of them opened in Gimp.
In my case, here's where I started from:
The first image:
and the second one:
Step 2. Select the image that will be the "base" one, i.e. the one you want as, say, "background". (In my case, this was the "Matrix" picture.) Next, select "Layer" -> "New Layer...", then give the new layer a name and select "Transparency" in the "Layer Fill Type" section.
Step 3. Naturally, now that you've created the new layer on the base image, you want to paste onto it the overlapping photo. To do this, go to the second photo and copy it ("Edit" -> "Copy") and then come back to the base image and paste it on the new layer ("Edit" -> "Paste").
Step 4. Now you've got the "top" photo over the "base" photo, in a separate layer. The problem is you can't do much on this second layer because, erm, well, it's "floating". You have to make it stay still :-) Go to the "Layer" menu and select "Anchor Layer" from there.
Step 5. Next, you need to add to your overlay ("top") layer what is known as a layer mask. To do this, select from the "Layer" menu "Mask" -> "Add Layer Mask..." and from the options present there, pick "White (full opacity)".
Step 6. Next, you need a gradient on this layer mask in order to "make" the transition between the two images. From Gimp's main window, select from the toolbox the Gradient tool ("Blend tool"). For the purpose of this particular blending, I chose a radial gradient, from light gray to an even lighter gray. The selected fill options are the same as in the image below:
Step 7. Now it's time to actually apply the gradient on the mask layer. I chose to drag a diagonal line in order to apply the radial gradient (such that the radial gradient could cover all the picture). Just drag a line (with the gradient tool selected) on the mask layer, from, say, the upper left corner to the lower right corner.
Step 8. Voilà. The gradient is now applied and the two images should be present in a "blend". The result should be something similar to the picture below:
Step 9. If you're not satisfied with the results, you can tweak various parameters and repeat Step 6. and Step 7. until you get to like the final result.
Step 10. In order to be able to save the resulted picture in an usable image format, you must have only one layer; that is, you need to "flatten the layers" present in the picture. From the "Image" menu, pick "Flatten Image".
Step 11. Now you're ready to save your new picture. Choose "File" -> "Save As..." and set a file name for your picture. (Just make sure you pick "Save As..." instead of "Save", otherwise you'd end up overwriting your original "base" image - in my case, the "Matrix" one.)
I don't know if anyone might find this useful, but as for myself, it's certainly a good idea to keep a good account of new learned "skillz". :-)