Grainy Portfolio Images

I’m trying to add images to portfolio items. They show up perfectly clear when you look at the entire “projects” list, but when I click to go into the individual item, all images are terribly grainy. Contacted godaddy support and they told me that the 4000 image was being shrunk down to 300. But he didn’t know why, said it was an issue with the theme. I can’t figure out how to make them display correctly. Please help!!!! I have to get this done by Monday and I’m having so many issues.