I found Ken Lunde's Home Page
which has an excellent selection of Firearm Wallpapers.
When you link the Wallpaper page, the links at the top are recent updates...scroll down to see the whole selection.
He has nearly 200 different pistol wallpapers. For you knife guys he has a knife page as well.
You can also Google image search the gun you're looking for,
then look for one that is a nice picture in a large pixel dimension.
The Python and Delta Elite are from Lunde Studio, the S&W 500 from Google.
One of the Lunde Studio knife wallpapers, Owyhee Hunter by Scott Cook Knives.