Hi Charlie; welcome to the Forum!
It is possible to attach photos up to 512K in size which can display pretty large depending on screen size. If you are using a PC and are using Microsoft's free PowerToys ImageResizer (or its equivalent for WinVista and 7), you can choose to resize "Large" and it comes out much bigger in dimension, though reduced in bytes.
Alternatively, if you already have photos stored online at a hosted site, you can link them in the body of your text with html tags and they will display close to full-screen in size. I've posted a little tutorial that might help here:
http://www.thorncycles.co.uk/forums/index.php?topic=4313.0If you get truly stuck, PM then email me and I'll reduce and post them for you as attachments.
Best of luck on your sale; that bike is a beauty.
Dan.