So... anyone have any idea where we're supposed to place them and what resolution they have to be, to be used in the editor?

Assuming you're talking about normal sign blocks, skins for them should be placed in ..\Documents\ManiaPlanet\Skins\Any\Advertisement. You should make sure to restart your game after adding skins there (you may be able to just hit the refresh button on the map selection screen, but I have not tried this) so that the game lets you select your signs. The sign dimensions aren't specific, I don't think, but should be at a 2 to 1 ratio. For example, the MX signpack signs are 512 X 256. This size should be sufficient for most signs.

If you're talking about other skinnable blocks, I'm afraid I can't help you.

That's the 2 blocks high, I would like to know about the 1 block high.

I hope you found the location by now. It's:

\Documents\ManiaPlanet\Skins\Any\CircuitScreen\

And the resolution can be 1024x256. The sign has to be cut into 3 signs. The top half can be one sign. The bottom half has to be cut into two signs side-by-side.