I haven't discovered a means for getting rid of it entirely, but I can get rid of the gallery by hiding it in the start bar.
Look to the portraits of people's images. Just above your image should be an arrow similar to ↗ (an arrow pointing upwards to an upper-right corner) in appearance. Clicking this puts the gallery into its own window. From here you can minimize it.
It's not perfect as it pollutes the start bar and all open windows selection, but it's the best that I've found. Be warned that they also made it one of the 'higher' priority windows, which is awesome. So you really will want to minimize it, and it should go away from your view until you unluckily click or open it again... all from personal experience.
Does
Options > Personal > Unchecking Show pictures of contacts
help? – Karan – 2015-05-06T08:44:28.6071No. Skype For Business still insists on showing big pictures of everyone in the voice call. – Jez – 2015-05-06T08:56:36.980