For web admin's info, it's possible to incorporate stylesheet switching using css/cookies and a drop down selector somewhere on each page - and then you can override the site's <img> tag with no images by using "display:none" in the "worksafe" css mode. this is a fairly pain free way of doing things and guarantees no images ever appear for the end user wherever the stylesheet dictates (not just sigs).
I did this on my car forum, and replaced the main images with an image reading "dictionary discussion" or some other nerdy title. ho ho ho. you can do quick "Christmas themes" and so on, and leave it up to the user if they want to use that particular stylesheet theme.
obviously it's up to you as to whether it's worth investigating this route, the sig disabling seems fairly thorough to me.
