Limiting Image Size in Entries

I'm using a custom style that someone else made using Minimalism. I'd like to limit the size of images in entries as a lot of my buddies aren't tech savvy and routinely post super large photos without a cut. I've been trying to help them out, but clearly I will need to do something on my end, and I'd like to avoid using image placeholders if I can. Could someone help me out?

I found this coding for Smooth Sailing and was hoping I could use something similar:

.entryText img {
max-width:600px;
}

Thanks for any help you can provide.