If Some of your photos are in landscape mode, and some of your photos are in portrait mode, this could cause a larger gap of space between one Content Chanel and the Photo carousel. Uploading photos in a single view (all portrait or all landscape) will prevent the problem. 

Another option is to turn on smooth height in the carousel style, to do this navigate to:
  1. onMessage > Website > Site Name > Styles
  2. Edit the Style being used on the Photo Carousel
  3. Click Edit Settings (Gear Icon) in the upper left
  4. Select Yes for Smooth Height On