Please, can anybody tell me how do I do this kind of decoration (on the attached photo)? Although Spider Man is flat, it looks like "convex". I have no idea how to achieve this effect and it looks really good. Thank you for every advice.

I don't think Spiderman is flat. I think he was modeled out of modeling chocolate or fondant but in sort of a 2D/3D approach.
Just looking at it for example the leg on the left was probably a cone of fondant that is tapered down into the foot and flattened on the back and placed on the cake. Then the body looks a lot harder but it looks like you could cut out the initial 2D shape and add a ball to the shoulder and then rub it into that area so the seam is less obvious. Since the spider web is so busy it may hide seams etc.

As silly says the leg & head are just larger cones or eggs of fondant (or could even be done w/b'cream). Yes, the majority of the figure is flat, but the head & leg are more 3-d. Just shape an egg size & shape for the head and sort of a cone shape for the leg.

It is a technique called bas-relief .
You build up the shape underneath first then put a thin piece of fondant over to finish. It gives definition without obvious seams .

I found a website of an author of this cake. And here is some tutorial:
http://www.fun-lovingcakes.com/spanish/photo_gallery/tu01.htm

So basically I have to make the decoration (Spider Man) with two layers of fondant. First layer is some kind of a "base" and then I put another layer in places I want to look like "convex". Right?

Yeah, you're sort of right The 'base' can be the flat part of the design (spiderman) and the next layer will be as we described above - make an egg of fondant for the head etc.