Open Photoshop. Go to Image and then down to Image Size and open the dialogue box. The resolution doesn't matter, but 300 will do. Make sure there are ticks in all three of the boxes on the left (Scale Size, Constrain Proportions and Resample Image.) Then type the required value for Width into the top rectangular box on the left. If the image is landscape format then type 1600 here. With Portrait orientation type 1200 in the rectangular box marked Height. (You only need to input one set of pixel size in either the Width or Height boxes as the resultant values will be automatically calculated.) Then click OK and save your image as a JPEG.