Trish,
Here's a picture of where I keep my water level on my skimmer :
I use a weir, and if I let the water get halfway down the skimmer, it'll start sucking air.
I looked at the picture of your pool, and it appears that your skimmer doesn't have one (a weir). You might try putting a tennis ball in your skimmer basket to help keep the pump from sucking air.
Here's a picture of where I keep my water level on my skimmer :

I use a weir, and if I let the water get halfway down the skimmer, it'll start sucking air.
I looked at the picture of your pool, and it appears that your skimmer doesn't have one (a weir). You might try putting a tennis ball in your skimmer basket to help keep the pump from sucking air.