I USUALLY CHANGE THE PIXELS TO 600 ON THE LONG SIDE WHICH USUALLY PUTS THE SHORT SIDE AT 400 AUTOMATICALLY
I USE MS DIGITAL IMAGINING SUITE 9 BUT MOST COMPUTERS WILL COME WITH SOME PHOTO EDITING PROGRAM AND YOU OPEN THAT PROGRAM AND OPEN THE PICTURE FROM THE PROGRAM. THEN FIND THE RESIZE OPTION. THEN SAVE THAT PHOTO. I USUALLY RENAME IT BY ADDING THE WORD "SMALL" TO THE NAME I HAD ALREADY GIVEN IT. JUST REMEMBER WHERE YOU SAVE IT TO SO YOU CAN GO TO THAT FILE TO DOWNLOAD IT. GOOD LUCK.