Is there no longer an option to change the default forum icon other than having to reupload the same image for each category?

No, wish you could, whilst it's nice for those that want individual icons, some prefer just the one like me. You can edit the forumRow template on row 9 and replace {file="$forum->icon" extension="forums_Icons"} with your image path

 

@daveoh thanks! The current forum icon system is a bit non-seo friendly atm so I'm opting out and instead using a static icon to reduce response time and raise my seo score.

@daveoh @iDeath

You should be using custom.css instead to prevent override changes while IPS upgrades. Refer to @steve00 method here: https://invisioncommunity.com/forums/topic/427212-forum-icon-help/?do=findComment&comment=2622108