After I choose a picture, how can I put it as my background?

i have an apple laptop so how do i set as wallpaper?

On the Apple menu you will have to click on Control Panels and the select Appearance from the available options.Your current desktop picture or pattern will appear in that window. You will have to remove the current one, so click on Remove Picture. Don't worry about losing it, because you can always get your previous desktop images back :) Click on Place Picture and then drag the graphic you chose to use as wallpaper in the preview window.The picture in the Preview Window will become your desktop background if you click on Set Desktop.. I hope that helps you.