I posted http://www.paperbackswap.com/Attack-Titan-2-Hajime-Isayama/book/1612620256/ and the image says "NBC India image temporarily unavailable". What does that mean?
 

you've stumped me, but at least it let you post the book.  Seems to me it's kinda like when the book cover isn't available and PBS just shows a blue cover instead.

It's another website's equivalent of our blue "no image available" box.  When the server imported the listing, it imported the white box with it, as if it were a book image.  If you have a scanned image of the book cover, we'd love to have it.  To post it, go down to the bottom left corner of any page.  Under "my account" click on "member tools" and choose "upload book covers."

Once your image has been approved, it may take a while to appear on the listing, because it's replacing a current "image."

Images uploaded by members are a major contribution to the site and are welcome.

In addition to member-uploaded images, PBS also imports or inherits images when updating its database. Sometimes these are very good, but sometimes they may be poor quality, incorrect, or images comparable to the PBS "No Cover Available" blue boxes, or may be fake cover images.

In any of these cases, members may submit scans, photos, or images they find on the web to replace "no image available" images, or poor quality and incorrect images found here. We welcome the assistance of any member who is able to help with this. All images uploaded to the site by members are subject to approval, and images may be denied if they are too fuzzy, too small to be legible, not properly cropped, or do not match the title and author. If an image is not accepted, we write to the submitting member to let them know why.

Also, note that there is an important distinction between the PBS "No Cover Available" blue boxes and "no image available" images imported from other sites. PBS will only display a blue box if we have no image at all for a book, but will display imported "no image available" images as if they were actual book covers. In terms of image display, there is no distinction between  true book cover images and imported "no image available" images.

Furthermore, note that due to the way internet servers are updated, images uploaded to replace PBS blue boxes will show up much more quickly than those uploaded to replace images already in the database, including imported "no image available" images.