If you are simply using RoboHelp to configure the size of the image, you are likely ending up with awful results. One should always consider using an image editing application to scale images first, then insert the resulting scaled image into RoboHelp and presenting it at full size.
However, if you are fine with the way things look, you could simply use CSS to configure a consistent size.
The way you have resized results in HTML something like:-
<img src="my.png" alt="" style="border: none;" width="350" height="315" border="0" /></p>
and it could be written across more than one line.
If you have someone in your team who can write regular expressions that will find
within the image source tags and remove just that part, that would return you images to full size.
Then you would need to create a style in the CSS as Rick has suggested and apply that to all images.
Finally you offer up a few chickens to ensure all the images look as you want.
See www.grainge.org for RoboHelp and Authoring information
How do you create a style in the CSS that configures a consistent size?
Do you have a recommendation on an image editing application?