What do you mean the image specs?
You shouldnt be uploading large images and then displaying them as thumbs by specifying small width and height parameters, it will make your site very slow and isnt good practise!
If you want a small image that opens up as a large image, make a thumbnail in photoshop, upload it and display it.
Make the image a link and link it to the larger image.
an example in RokBox (below)
Here we have 2 images
THUMBNAIL:
demo.rockettheme.com/extensions/images/s...box/sshot2_thumb.jpg
FULL SIZE:
www.rockettheme.com/images/extensions/joomla/rokbox/sshot2.jpg
{rokbox title=|Vauxhall VXR8| size=|640 594| thumb=|images/stories/demo/rokbox/sshot2_thumb.jpg|}http://www.rockettheme.com/images/extensions/joomla/rokbox/sshot2.jpg{/rokbox}