With the screen image showing on screen (thus, you are in Paint), click the File tab, then Page Setup on the dropdown menu.
You see a grey "envelope" that represents the position of your image on the screen. This image will probably not be centred within the broken line frame.
Click Orientation to set the image vertical or horizontal.
On Scaling (lower right of Page Setup dialog box) you can see Adjust To and Fit To.
Click Fit To button and you can then alter the figures in the two Fit To --- by --- boxes. With every alteration of the numbers, the "envelope" moves its position within the broken line frame. Alter the numbers until you get the best position of the "envelope" within the frame.