Gray box/border around Photo Blog posts

Hi

I am hoping someone can help – If you have a look at the website it may make it easier to describe. Scottish Moments

When posting a photo in the blog post it has a gray border around the photo and also underneath. I want to remove this/ change to white.

How do I do this, I have tried a few things.

I have changed most of the other gray boxes and background to white but this part I cant seem to get rid of.

Thanks