You need texture (pic.) in format 256x128 or in .paa-.pac and go to "Load Texture" and when you are selected point what you wants to by a textured press "B" and they will textured!
HOW TO MAKE .PAA AND .PAC FORMAT??
You need TexView Program for OFP and ArmA , To you make texture in .paa , .pac ,format of your texture needs by a 256x256 and you only needs to convert they with "Save As" > "texturename.paa" or "texturename.pac" and you have it