Check through all images on the Analysts Guide for alt text / aria-hidden

**Describe the bug**
All images on the analysts guide should either have alt text or be hidden from screen readers if they're decorative. Each image needs to be checked and assessed, and either hidden or have alt text added as appropriate.

**Expected behavior**
Screen readers can work with the site appropriately, in a way that is useful to people. It is worth testing each page that's been changed with a screen reader to see how the user would experience the page after it has been modified to make sure that the correct option has been chosen each time.

**Additional context**
[You can learn more about decorative images on the W3 site](https://www.w3.org/WAI/tutorials/images/decorative/).
W3 also have [information about how to deal with complex images, e.g. graphs](https://www.w3.org/WAI/tutorials/images/complex/)

No files or tests are named. Start by locating every image in the Analysts Guide and reviewing each page with a screen reader; classify each image as informative or decorative. Done means every informative image has suitable alt text, every decorative image is hidden from screen readers, and the changed pages have been checked for a useful reading experience.

Written by the indexing model from the issue text.