Thanks to all, but you've all just confirmed what I thought - the source shouldn't be modified when I save, but it is.

If I put in the code exactly as tpartner suggests above, it just wipes the "max-width" bit and ignores that I've ever tried to put it in (and the image doesn't resize).
I've attached the pre- and post- save screenshots (LimeSurvey version 2.06).
Is it possible there's something in the css that is overriding??
I'm feeling like I have to think of another work around as the survey isn't going to work on mobile devices if I can't get the image to resize properly!!