Hi Val, & Welcome to Tom's Hardware?
As you are finding out, there are differences between Win-XP and Win-7, and it's setup and configuration.
First thing to do is check to make sure the screen resolution set up matches the native resolution of your monitor. Right click on your desktop, choose 'Personalize', then display. Should be set at 100%. Then back on the desktop, click on 'Screen Resolution' and confirm that the res chosen matches the native resolution of the monitor.
Are you using a specific photo viewing program, like photoshop elements, or paint, or paintshop pro, or the native Windows Photo Gallery?
Open the pictures Library, where you should have your photo images stored. Either double click or right click ' preview' and the photo opens now in Windows Photo Gallery. (Used to be Windows Picture & Fax Viewer) It should adjust the image size to fill out your monitor area. You can increase or decrease the zoom factor at the bottom, but each image chosen will open at 100%.