Easy Slider picture problem

I put two of my own pictures in easy slider. There is still one default picture in the easy slider. One of my pictures came out very similar in size to the default picture. It looks like the program cropped some of the picture off to make it fit the slider. It still came out just a little bit taller than the default picture in there, so the box moves a little when that picture comes up to compensate for the picture being a little taller in height. The other picture I put into the slider came out alot taller than the default picture. When the slide comes to that picture the box moves alot to compensate for the much greater height of the picture. I don’t understand why this is happening. I thought the picture size for the slider is set in the theme. So, when a picture is inputted into the slide it should automatically be adjusted by the theme program. Thanks for any help.