Hi there,
This behavior is totally normal. Resizing your picture to be exactly 150x188px would make it look ugly and deformed.
What Hikashop is doing is resizing your pictures to create a thumbnail which keep the original ratio, so it make the width or height fit the configuration and resize it.
Therefore, I don't know what you would like for your pictures, make them fit your exact size? Or maybe you would like them to be cropped?
Anyway, both these options aren't easy and it's not said that it will look better.
If you have any question, do not hesitate .