Oh thank you very much for the interesting advice! I didn't know about the EPSs usefulness. I'll get more infos thereupon as well as for the other things you said.
Yes, you're actually right. Inkscape and Gimp can well reproduce those examples but as for the easiness and accuracy (especially when it comes of 3D perspective* and distortion of the images) with which the Adobe suite can do something like this https://youtu.be/I9_SR02HQIE there is probably noway to get it with the alternatives.
Thank you very much!
* In the past I've been using Paint.Net (http://www.getpaint.net/index.html) on Windows to obtain a realistic 3D tilt of images (the distortion instead is probably not possible with this program; I don't know). No other program so far has been able to reproduce something similar in such an easy way (surely Blender does but it must be somehow more complicated). When I had to shift to Ubuntu, I couldn't use Paint.Net anymore and got lost!