Keeping <li> images the same size

Hi guys,

I think this is probably a very basic question, so apologies in advance! I've started on the front-end web dev track and have completed the build a basic portfolio section, and now the Lake Tahoe CSS redesign.

For practise, I wanted to remove one of the placeholder portfolio images we use on the course, put my own in its place and link it to my Lake Tahoe project. So, I've knocked up a quick image in Photoshop and put that in, but it's displaying as slightly bigger than the other images we put in.

You can see what I mean on my website; http://codedbyrob.in

I was hoping the responsive styling we put in place on the course would resize it down to the same size as the others. As it hasn't, is there any way I can do this in code?

I wasn't sure the best way to show you the relevant code, so I've created a GitHub and uploaded my portfolio homepage, the css styling and responsive css doc. Link is https://github.com/codedbyrob/portfolio.git

Thanks so much for your help

Rob