I am developer but I am in a situation I should act as a designer too for awhile..My question is I had an image I want to it to be a background for an app so I need to modify it to something like this app's background; it's not blur but a little darky..I need to know how to add that layer to the photo with simple tools?
One way to get a similar result is creating a gradient and placing a low opacity image over that background.
Create a colored gradient background using the Gradient Tool (G). I picked similar colors from your example image.
Place the image on top of the gradient background layer and set the blending mode to Luminosity. Lower the opacity of the photo to ~25%.