Well, Auctiva seems to do a very good job in making multiple images displayed together have equal height, i.e. they size them to max height and display well in that click to supersize mode.
When reading your initial post, it sounded like you were trying to move your pics to Auctiva storage and (I assume) that was in part to use the other features, like templates. That's why I was recommending you let Auctiva do the resize.
If you want to continue linking to external, then sizing them for display would be one method (which seems to be difficult), or the insert image feature icon (top row next to last on toolbar) in the lister does have a sizing feature for image that forces to a width or height limit and you can see the result immediately. Just type in 300 for height and it automaticaly resizes (note lock icon to keep proportions correct).
Danno