Could you please double-check the DLC folder name and confirm that it was installed correctly? You can use the image shown on this page as a reference, but the folder should be called "14 Nights With You" and not "14nightswithyou-paid" (or "14nightswithyou-free").
I assume you're getting this error because the file path for the image folder starts with the "14 Nights With You" folder. If it starts with anything else, Ren'Py will ignore it and act like the image can't be found ^^;
If this doesn't resolve your issue, please let me know!!