Many functions in Photogather require you to select a photo first to tell the software which picture you want to apply this function.

If you want to make a single selection, just left-button click the photo you want. The photo will be lightened after being selected.

If you want to select more than one picture, press left button of your mouse and do not release it, move the cursor. Now you will see a transparent rectangle. Drag the transparent rectangle to cover the pictures you want to select. After releasing the mouse button, all those photographs will be selected. You will see they are all lightened.

If you want to make more selection, do the same as above while pressing [Ctrl]. Then the new selection will be added the previous selection.

If you want to cancel the selection, just left click on the desktop.