Is there a list of Salesforce images that can be used in custom pages and formula fields?

I would put using images that salesforce use to render stuff in the same broad bucket as screen scraping.

Whilst this may work okay, there isn't a way to warranty against breakage some day.

(I've used the sfcti js resource for CTI and the locations have changed twice in the last 12 months, there isn't a way to tell when they changed except for when users discovered CTI won't work anymore, therefore this approach surely has its perils)

Having said all this, a quick google search threw up this page which seems to catalogue quite a few images / icons.

http://www.force2b.net/index.php/2010/08/salesforce-icons/

You might also check out graphics pack from Salesforce labs on AppExchange. Lots of examples and icons: link

Well, I was also referencing those links earlier and found that some of them no longer work and others don't have all the images.

You can still look at all the CSS files provided by Salesforce and refer images within them. I just did it and compiled it on my blog here: http://www.vermanshul.com/2017/10/quick-tips-salesforce-default-images.html