Website not showing properly when customer logged into account

[toystoyou]

I've just started using customer accounts and have discovered that when a customer logs into their account my website doesn't display images properly and the layout is totally different to the regular layout.
I'm using V9. I've checked the knowledge base and trawled through the community search pages but can't find anything on this.
Can anyone help?
Thanks.

[NormanRouxel]

As always, an URL is needed so we can see what's happening. And a test login name and password.