Image size on workshop items with product block

I am just setting up my website. We are running workshops so I have a workshop page, and then a product block so that people can book. The images on the  website are huge and instead of a thumbnail they are the full width of the webpage. Is there a way to make these in to thumbnails instead of being so big? We are going to have lots of workshops so I could really do without the images being so big! I've attached a photo to show what the website is showing. It's so big you can't see the whole image without scrolling down.

Hi. Can you share link to page in screenshot? We can help easier