Most of your images will have more pixels (for printing and making slides, the more pixels the better). To enter a Society projected image competition you must reduce the number of pixels to 1600 wide by 1200 high. You must not have the width more than 1600 and you must not have the height more than 1200 pixels. If for example you reduce the width to 1600 pixels, and the resulting height is 694 pixels, then you need to add some black pixels to the height to make it 1200. In Photoshop, go to "canvas size" and make the height 1200 pixels, background colour black. A band of black will be added to top and a band of black added to the bottom, to give the 1200 pixels.
For images in "portrait" mode, with 1200 pixels high, the width pixels will be less than 1600. In this case, bands of black need to be added to left and right, to give a canvas width of 1600.
The bands of black, do not have to be black. They can be any colour or pattern, but black is usually the preferred colour.
To reduce the size of your image. Go to "Image". Go to "image size". tick the 3 boxes at the bottom (scale styles, constraint proportion, resample image). choose "pixels" from the drop-down menu.
The images below show examples of the black bands.