Edit>Map Settings (ctrl+M) > choose texture (all are stored in Soldat/textures/...) Remember not all file extentions are supported. If u cant find your texture it probably means you should change its format.
Regarding sceneries: hit R to toogle the Scenery tool, then RMB on the map to open a scenery menu -> select the desired one and you can place it with LMB. Additional useful scenery features will show up in the Scenery window, keep an eye on it. Same as with texures, all are stored in Soldat/scenery-gfx/...