4
This should help you...
Create the images in whatever graphics software you are using to the exact dimenisions (in pixels) you want them to be on the web. If you want pop-ups, I would suggest using two sets of images, one for the thumbnail and one for the pop-up. If not, at least make the two pictures the same ratio (length to width).
After you have the images created and uploaded to your webserver, reference them in the html code with a line like this
. The key is the height and width attributes. Without those, the pictures can be distorted, which is what you are reporting. Possibly, those numbers are just wrong or inverted as well.
Proper XHTML would call for the alt, which is also good for blind people, and for the / at the end of the element.
If you are using web creation software and don't know the underlyin code, you may be able to specify the dimension somehow, I don't know. Try it and see!