There aren't exact dimensions we can give due to the way the slider is programmed. However, what we can tell you is that the viewable area on most designs is 360px tall and that whatever image you insert, the middle third will be viewable. Most of our images that we use as default are 1200x800 but only the middle third is viewable in the slideshow. One way to adjust your image so that it fits is to put a transparent png file that's 1200x800 behind your picture and simply insert yours into the middle of that transparent image.