If you have photoshop, open image in photoshop and click Image/image size and enter 800 in the box for pixel dimentions.
If you have no software, download Faststone Image Viewer from
HERE.
Open up the image in Faststone and click Resize/resample from the left hand menu and enter 800 in the Pixel size box.
A small tip, only fill in the 800 px in the longest side box and this will retain the aspect ratio.