Nick -
It's hard to say without seing it, but it sounds like your
image size is defined as a smaller size or is in a fixed-size
table. We could tell you for sure if you could post it where we
could see it.
If you are using RoboHelp for Word, three hyphens and
[Enter] creates a horizontal line that translates to an html
horizontal line - the <hr> tag (as long as something else
follows it). Otherwise, in RoboHtml, you can go to the menu option
Insert a
Horizontal line, which is the same <hr>
tag.