Can this be done?

Let's say I add an image to the start script using the usual 'Add picture' script. Could I then insert a line of CSS into the UI to fix the image in place?

You would need to be able to work out a CSS selector to use that would reference the picture. You might be better off emitting the HTML yourself (via OutputTextNoBr) and including the id or class you want to identify it to the CSS.

Thank you. I don't fully understand this, but as it's related to another (now resolved) problem, I'll get back to you if needed.