Gallery pics too big. Resizing question

Here’s my site.

I’ve searched all the forums, but can’t seem to find an answer. I’m sure it’s an easy one, being an HTML noob, I need help.

The pics look fine in the gallery, but when you click on them, they’re HUGE. How do I resize them to a standard size without editing each one individually? Is there a plugin (that’s not 3 years old?)

Thanks so much.