You can do this very easily in Aperture. First in Photoshop (or whatever application) make your logo/text. You'll probably want to make the white areas transparent; otherwise youll have a white block on your photo. I have black text with a fuzzy white border on a transparent backgroundthat way the text can be read over pretty much any photo.
Then, in Aperture, set up a new export preset (Aperture > Presets > Export Presets
). Alternatively, you could edit one of the built-in profiles (but I tend to want to leave those clean). Set everything the way you want it (perhaps you want to export a full-size JPEG with the sRGB profile), and then you can choose your watermark in the section at the bottom of the sheet. Experiment with the settings. Try exporting an image or two using your new export preset to see if youre happy with the resultif not, you can of course go back and fiddle with the settings until you come up with something that works and youre otherwise happy with.
NB this thoughif you want to change the watermarked image at a later date, it will not suffice simply to re-open your watermark in Photoshop (or whatever) and overwrite the file. Aperture doesnt just look at the file every time you export: when you made the export profile it opened the image and saved it into its own settings. Therefore, if you change the image, you will need to re-load it in the export profile settings.