hello , where can i find nice background for my game like this one?
There are hundreds of sites that provide resources, you just need
to search for them (make sure the license allows you to use it), but I don't know if you'll find something like that...
If you want exactly what you've go in your mind, then get an artist to do it.
and should i put it on the Window for all the level (i mean to make it size 800, 3000)? or try to add it to the camera so it will keep the same 800*600 picutre ??
It all depends on your game. If you have just one fixed background per level, it probably would be better to have one separated image for each level, but if the player can move around on the background it's probably better to have one texture and move the sf::View (aka camera) on top of it.