If you are having a website with a sidebar (left or right hand side menu), it is important that the height of the images are all the same. This keeps the arrows in the same place as you click through.
If you are having a website centered (without a sidebar) it is important that the widths are the same (again, so the arrows stay in the same place across a slideshow).
For sidebar sites, a height of 495 pixels (any sized width) is recommended. 495 pixels high will usually allow a whole image to appear on one page, without the viewer having to scroll down to see it all. It is also big enough to look impressive, whilst not causing too many major problems with loading times.
For non-sidebar (centered) sites, we recommend a width somewhere between 330 (min) and 400 pixels (max) wide. Pick one width and apply it across your image (any size height is ok).
If you would like an oversized (big) image, for a title or home page say, send it as 1600 pixels wide x 1233 pixels high (if you would like it to fill the whole screen), or 700 pixels wide x 625 pixels high (which is not whole screen, but a nice, big, single feature image).