Ink just loves big images. Use them as big as possible, preferably with 1280px in width, don't worry about the rest, InK will stretch and resize them accordingly to the available screen size.
If you need technical information on how to start your Ghink website environment, head up to our Github repo.
Otherwise, read the example blog posts on the boilerplate to find how to use this framework and extend it to your needs.
Ghink is heavily based on InK. We suggest you look it up first before you start, to understand the basics behind it.