Putting tick mark in place of bullets in unordered list in RoboHelp HTML?

You'd need to download (or create) an image and specify its path in the Bullet Image field of the Bullets and Numbering dialog. However if you want to add an image to your CSS file you'd have to do things differently. As I am not a CSS expert I'll leave that to others, although I'm sure this has come up elsewhere on the forums so you could use the search.