Photoshop :: Add Rectangle Shapes To An Existing Image

I need to add rectangle shapes to an existing image.

I tried using the rectangle tool but each time i click on the background option after the rectangle is drawn, it disappear.

The rectangle will only appear again when i select the rectangle shape from the layer panel.

Is there anyway where i can make the rectangle show on the image?

i tried reading up some of the thread posted and i saw some thread mentioning doing something to the path. How is the path used?

I'm not really familiar with using paths and layers, so i need a really layman's term of explaination.

For more info, i actually set the colour of the rectangle fill 100% opacity 100%.

anyway wat is the purpose of setting opacity?

cos i tried ranges between 0 - 100 and there doesn't seem to have much difference.