Replacing and adding artwork in Verdant Skies is pretty easy- the hardest part is figuring out the name of the file you want to replace.
Create an "art" folder inside your mod. Use the PNG format for any artwork you want to replace. When replacing artwork, just make sure it has the same name as the artwork you're replacing. Otherwise it will be considered new artwork available for you to use in your mod.
Replacing or Adding Portraits
Portrait artwork is the large image that appears when a character is talking. It follows the pattern: <name>_<emotion>.png. For example, "Emma_sad.png" to replace Emma (Rosie's) art, or "Marina_smile.png" to add art, as in the Marina example mod.
Each character needs art in all 4 directions (Up, Down, Left, Right), with 7 frames in each direction. Frame 0 is standing and facing in one direction, and the other 6 are walking frames. They should follow the pattern "<name>_Walk_<direction>_0n.png" such as "Marina_Walk_Down_02.png".
Any other art can be replaced directly also. For example, the Free Money mod uses replaces "glitchPainting.png", so it uses an existing painting name (and prefab) to add one worth more money and with custom artwork.
Finding Art Names
To replace existing art, you need to find the art inside the base game. Take a look at the base content in the modding resources http://verdantskies.com/modding-resources/ at the developer's site. There are a few oddities to be aware of. Jade is named Hailey, and her artwork is spelled "Haily" for her walk sprites. Rosie is referred to as Emma. So you'll want to double check your assumptions.