Actually just changing the resolution to 300 doesnt work.... what you need to do in Photoshop is UNCHECK the "Resample Image" box under Image size in the Image Menu. Based on the size of your image- that would be a 5x7 at 300dpi or an 11x7.5 at 200. A 16x24 (based on those proportions) would be less than 100dpi- which usually isnt recommended for printing.