Watermarks are those faint, transparent designs that websites often use to mark exclusive photos when breaking a big story. For example, many of the great fake iTablet "photos" that we've received during the past few weeks have been emblazoned with a watermark for one Mac site or another. Impression (€9.90 -- about US$14.10) works in iPhoto to put watermarks onto your iPhoto pictures. If you're worried that a watermark might ruin a perfectly good picture, don't be. Impression makes a copy of the picture, then creates a watermarked version which is also saved into your iPhoto library.
Like iPhoto2Twitter, Impression works through the File > Export command. Once installed, Impression has its own pane on the Export settings panel. Select a photo or group of photos, Choose File > Export, click on the Impression button, and then use the buttons in the heads-up display. Those buttons allow you to select a PSD, PNG, or RTF file containing your watermark, rotate the watermark, or change the margins, transparency, and location of the watermark.