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

[jen-machin]

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.
W3 also have information about how to deal with complex images, e.g. graphs