NivoSlider crops images to weird results

Hi there!
Situation:
I use the "Brick and mortar"-Wordpress-Theme (https://themeforest.net/item/brick-mortar-a-personal-business-theme/1939048) with the NivoSlider-Plugin. On the single post page, the featured image will always be cropped to 715x214px.

Problem:
My client would like to be able to upload different sizes (also upend) and not have them cropped in the process (weird results sometimes as there is no way to determin the cropped area).

Question:
How/where do i tweak the plugin in order to be able to choose if i want the fotos cropped or not (i´d rather have them not-cropped).

Help would be very much appreciated!

-
Thank you!