1. A non-transitory machine readable medium storing a program for execution by at least one processing unit of a computing device, the program comprising sets of instructions for: displaying a movable user-interface tool for applying either a first effect or a second effect to an image, wherein said first effect adjusts an attribute of a first set of pixels in the image according to a first gain function and said attribute of a second set of pixels in the image according to a second gain function, and said second effect adjusts said attribute of the first set of pixels in the image according to a third gain function and said attribute of the second set of pixels in the image according to a fourth gain function; in response to a command to move said movable user-interface tool in a first direction, applying said first effect to said image; and in response to a command to move said movable user-interface tool in a second direction, applying said second effect to said image, wherein the displaying and applying are performed by the computing device.