I need to configure the preview image with a smaller size than original, has a resolution of 1024 * 768px, I need to display with a resolution of 800 * 600px, and if you click on it in a new window to display full size. Is it possible?
If your referring to your medium images you can set a new medium threshold in upload options in admin and rebuild your medium images under scan database in admin
Preview Image 
Linear Mode
