[Evelyn43]

I've tried to add a cover image using the instructions in the Sigil tutorials:

You can add images to your book in several ways:

Use the menu entry File→Add→Existing Files
The images will be added to the Images folder in the Book Browser. However, they are not yet actually used anywhere in your book – they are just available to use.
Once the images are in your EPUB, you can insert them anywhere in your files.

Click on the location in your file where you want the image to be located (in either Book View or Code View) and then click the Insert Image icon in the main toolbar.

And also tried to follow the special instructions regarding covers.

Create a cover.html file at the start of your book and then insert your image into it.
Mark the cover.html file as a cover file by right clicking on it in the Book Browser Text folder, selecting Add Semantics and choosing Cover.
Mark your actual cover image file as a Cover by right clicking on it's file in the Book Browser Images folder, selecting Add Semantics and choosing Cover.

I have two problems. I was unable to mark the cover image as a cover. Right clicking on the file in the images folder did not bring up the menu where I can add semantics.

Secondly, I've tried this process twice and attempted validate the file. Both times I get the message (referring to my cover image):

This resource is present in the OPF <manifest>, but it's not reachable (it's unused).

Can anyone translate that for me and tell me how to correct the problem?

Thanks!