You can change the resolution for the JPEG file with practically any image editor, but it will reduce the quality. For example, you can resize a big image to a small resolution and keep the quality, but it's impossible to do it backwards.
To change the resolution, download and install XnView (http://xnview.software.informer.com/ ), then right click your picture and choose Open With. Select XnView, then go to Image > Resize and input the desired resolution.
Use this image as guidance: