Without knowing what software you are using to edit your images, it's hard to give specific instructions.. But make sure you save the image in JPG format. When you do so, you should have a 'compression' option or 'quality' option available somewhere during the save process..
Rather than changing the pixel dimensions, just change the compression (or quality).. Raising the compression level, (or lowering the quality level) will reduce the file size of the image.. Since the iPad 'stats' image is a simple, non photographic image, setting high compression (or low quality) won't have much effect on how it looks.