Cannot remove grey background for post thumbnails

There is a grey background on every featured image I upload. I found one post on the forum which seemed to address this issue, but it did not remove the grey. This distorts every uploaded featured image that has a white background. Is there a way to remove it? Tried so much, but apparently I do not have the correct code. Thanks for your help! Website is here. Work in progress, but you can view two pages for an idea of problem. MCenter & Immersion

Iz