Lens flare is one of the cheesy effect I just couldn't left alone. In general, lens flare is something photographers tried to avoid at all cost, but including it in digital images has similar effect to adding noise or shaking camera in Hollywood movies - to add fake realism.
To add it, just click where you want the center (which is usually an existing or desired light source), hold the mouse cursor and drag to size the flare up. This creates the main flare but other small "lens" flares and rainbow will be added diagonally over the whole image.
Lens flare can be also added partially or complete outside of the frame and it can be combined with stage lights.
Image can have only one lens flare so adding new flare will remove the last one.
To completely remove flare press DEL or just click somewhere on image and release without dragging.