For Catherine :)
Hope this makes sense, it's how I do it. I now have a folder full of different sized templates with different shapes.
File - New
Choose your name. Make it descriptive so you know what the template holds. Write a size in cm as I use the photoshop rulers in cm.
Set resolution 120 or 72 for the web. 240 or 300 for print.
Choose a colour background that will frame your photos.
At this stage you can save your template as a photoshop file to use anytime you want, with different photos. Save it as a photoshop file so it keeps the guidelines and different layers.
You can size the photos so they have a white gap/frame surrounding them. But I'm going make them flush. These are just random photos in my web file as demo, that's why some have my logo on them.
Layer - Flatten Image - save as a jpeg and this is my final image. Remember can you go and reuse the photoshop template file with different photos another time.
Hope that was helpful.