I tried to put these little signs onto the wall, that give hints to the players, e.g. like this [http://images3.wikia.nocookie.net/cb2 ... oxhurt.png](http://images3.wikia.nocookie.net/cb20090504155127/half-life/en/images/1/12/Signage_overlay_boxhurt.png) or this [http://images3.wikia.nocookie.net/cb2 ... t_jump.png](http://images3.wikia.nocookie.net/cb20110613225458/half-life/en/images/4/49/Signage_paint_jump.png)

But the editor doesn't show most of the signs, there's only a white (or black) empty square.

Some of the signs are shown in the editor, but they don't appear ingame, there just a black square instead. Look at this screenshot: http://files.homepagemodules.de/b29324/ ... 77n239.jpg

Am I doing something wrong?

I thought you need to design the entire sign as one thing there is a tutorial somewhere on this site have a search.

Those textures you are trying to put on the wall are overlays. Any use other than an overlay will make it glitch out.
Unless they were custom made by you, which could be a conversion/.vmt error, they should appear fine if put on a brush (not a func_brush).