Move the cursor over an opaque part of a page based graphic. Click the mouse button to select the graphic.
To select the image behind text, Command-Click (i.e. hold down the Command key while clicking) on the image. Command-Click will also let you click through an opaque part of one image to the opaque part of one behind it.
You can click through the transparent parts of an image (TIFF and EPS) to another graphic. You can see what parts of your graphic are opaque and which are transparent by selecting the graphic and setting a fill for it.