Custom Coding to resize thumbnails

Site URL: http://www.stevegallagher.com/

Hi Guys,

I was wondering if someone can help me out....

I'm working new page and I'm having trouble trying to scaling the thumbnails to the correct size. I would like them to be the size of the blue box but as default the page has the grey box set to a specific size. Also to adjust the padding around it.

I'm using the New Page > Images> Horizontal Masonry.

How can I write a code and place it, just for this page to turn the grey box's into the blue size? (there will be just 3 images per row)

See below: Thanks in advance

Blue box image size is H: 181 pix  /  W: 322 pix.  If that helps