This page suggests that setting the Favicon should be straightforward, except I cannot get it to work!
https://tiddlywiki.com/#%24%3A%2Fcore%2Fui%2FControlPanel%2FWikiInformation
I have tried various image formats in the _canonical_uri field (png, jpg and bmp) but none are recognised. Do I need to convert my image to another format? As there size requirements. I assume the image can be added to the wiki in the normal way.