forcing images that go into a slider to be specific dimensions

Hi,
I have support access on for aficreditsolutions.com. It has a slider on the home page and according to the theme developer the me dimensions have to be the following:

The standard size must be 500px * 300px i.e.,
width: 500px;
height: 300px;

Is there any way that I can quickly and easy force any image I put in the slide to fit those dimensions? Are there plugins to do this?