Steven Zahm@ Dani
You need to set the medium image size Ratio setting to crop to eliminate the adding of the margins. Right now it is set to the Fill option. The Crop option will crop and resize proportionally to best fit the specified dimensions, maintaining the aspect ratio whereas the Fill option will resize proportionally to fit entire image into the specified dimensions and add margins if required.
After you make this change, you would have to re-upload any images for the new settings to be applied.
With that said … maybe you might be interested in using a beta of the next version of Connections. I mention it because the entire image handling/processing has been completely overhauled.
Images are now displayed responsively and, and with a compatible device 2x (retina) images will be served. The cool bit here, these images are created, cached and served on demand using the new HTML5 img srcset attribute.
Additionally images size changes are applied on demand. No longer do you have to re-upload images to have image size changes applied.
Lastly, there is a new Fit Crop Mode (formally the Ratio option). The Fit crop mode will resize proportionally adjusting the size of scaled image so there are no margins added. The entered height and width are the maximum the image will be in either direction.
If you would be interested in giving a go, I would need a temp admin account and FTP account so I can install it and fix anything if something goes wrong for any reason … I do consider the code production ready, so it should be completely safe. Just post, privately, the login details here. Please follow-up with a public reply because the plugin I’m using seems to have a bug where I do not get notified of private replies which I have yet to track down and fix.