1. DPI? Min 300DPI
2. What format is best? TIFF? Just JPG with lowest/no compression will do the job
3. Which software would be best? I have Photoshop, okay?Yes... that would do the job. However, if you like, you may set a blank page to 4" x 6" (4R size) and overlay 6 passport photos on it. For info, passport photo size is 4.5cm x 3.5cm, 300DPI. Thus only cost you $0.50 for 6photos.