One way to shrink image size is to specify smaller width and height propotionate to the original width and height.
For example if you have an image
<img src="someimage.jpg" width="50" height="100"/>
Then to display a smaller image you would put it this way:
<img src="someimage.jpg" width="25" height="50"/>
But if you want to physically make the image smaller then try an Image utility like Photoshop.
I don't know if there's a way to create thumbnails with Java or JSP. My gut feeling says there probably isn't but I could be wrong.