Invalid input: img/books/400_front_thumb.jpg. Please provide a comma-separated list of id values, or a single id value. An id value is a positive whole number.
"First, Karl had to download and install a package called pgf. Then it turns out that inside this package there is another package called TikZ, which is supposed to stand for "TikZ ist kein Zeichenprogramm". Karl finds this all a bit strange and TikZ seems to indicate that the package does not do what he needs."
Invalid input: img/books/400_front_thumb.jpg. Please provide a comma-separated list of id values, or a single id value. An id value is a positive whole number.