This is the Pages for Mac support community. Preview is an inherent component application of OS X.
Preview no longer gives you access to annotated font transparency. This was present in Preview 5.0.3 on Snow Leopard.
You do still have access to the Apple color chooser via the Font colorwell in the Preview markup toolbar, but the opacity slider has been removed from the color chooser. Apple taking tools away again. 😟
Basically, in Pages you want to insert an image, and a text box. Place your text, and then with the text box (not the text) selected, you regulate the text transparency via the Apple color chooser opacity slider, or directly from the Pages '09 sub-toolbar. On either object, you want wrap disabled, so that you can slide your semi-transparent text over the desired image position. Select both and group.
Copying to the clipboard from Pages '09, and then choosing new from clipboard in a current Preview will give you a PDF image. Any version of Pages v5/v6 will give you a 72 dpi image (because of the transparency) that won't scale very well when enlarged.