There's a website called picresize.com
You choose the pic you want to resize from your harddrive.
Go to step two and choose custom size.
Type in 800 pixels. Leave the height blank, unless it is a lengthwise, portrait orientation, then type in 800 in the height instead.
I think you're okay to save as JPG, Best type, but if the file size is too large when you go to upload here, then just use "better."