The Yoast SEO plugin isn’t expected to touch or alter your image files. You may need to perform a conflict check. Please refer to this kb article: https://kb.yoast.com/kb/how-to-check-for-plugin-conflicts/
Hi Rumejan, thanks for the reply.
Yes I’d already identified that there is a conflict with the parent theme Kichu. Hard to find exactly where though, and can’t just replace the theme!
Instead I’ve installed this alternative plugin
https://it.wordpress.org/plugins/disable-attachment-pages/
It seems to have done the same job without breaking the link to the hamburger menu image.
Thanks for helping me troubleshoot this issue!