Moved 'contributing' images to their own directory

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | 2.3+
| Fixed tickets | #3192

The PDF version of the "Contributing Guide" doesn't show the images (as reported in #3192). The rest of the PDFs correctly show them. Although I don't know the exact cause of the error, the "Contributing" guide images weren't stored in their own subdirectory inside `images/` and named after the book. That's the only difference between the right and the wrong PDF files.

For compatibility reasons I've left on purpose the original images at their location and just made a copy in the new directory.