Can I use the normal desert background image in Mojave dark mode? I like most of dark mode in Mojave, however I much prefer the light mode 'desert' image to the blue-grey one that appears in dark mode. Is there a way I can use the light mode image when I turn on dark mode?
Yes. Also you can use the dynamic mode where the image is changing according to the clock time of the macbook. 
The wallpaper can be change independently on top of the mode that you are working on.