Two ways of doing this.

One - have a second slide for each image you want a pop up for. Add a hyperlink to the image on the first slide such that when clicked, it takes you to the second slide where the label is shown. Have a back button with a hyperlink to the first slide.

A more elegant solution is to have the label appear next to the image. You can add an animation for the label to appear.
1. create the label for the first image
2. Go to the animation tool bar and select your appearce animation - say, "Appear"
3. Go to the "Trigger" option on the tool bar (has a little lightning stroke).
4. Choose "On click of" and select the picture for that label

Now your label will appear when the picture is clicked.

But waite! now your screen will be covered with labels - probaby not what you want. So

1. from the animation tool bar, selelct the animation pane (your best friend when working with animations). Your "Triggered" animation should show in the pane on the right of your screen.
2. Right click it and select "Effect Options"
3. Select "after animation" and select Hide on next mouse clicki