Hi and welcome to MobileRead.
If it says that you have a file that is present but not reachable, it means that the file is not being used in the book.
If it's an image file, then all it's doing is taking up space in the book and you could probably delete it.
I'd load the book into Sigil and then look in the Images folder and look at the image to see what it is and do a search for something like:
Where xxxx.jpg is the name of the file. Remember to search all the HTML files.
If you can't find it, then that is why you got the message and it isn't in any of the pages so you wont see it on screen.
It might be a nessessary file, in which case report back that they haven't included the image file in any pages (only stored it in the EPUB) and let them add it. and revalidate it.