Please open the console (SHIFT+O) and type:
Do you see the game folder in the output? The one containing the images folder? It should look something like:
Code:
[<...some paths to optional content, if any>,
'<summertime folder>/game',
'<summertime folder>'/renpy/common']
If it does look like that, try disable the optional content in the configuration UI (parameter NO_OVERRIDES), start the game again and see if that helps. Check the searchpath again to be sure that no optional content is in the list.
Also, if the problem persists, we should move this discussion to the Mod's thread.