I would guess that maybe your theme adds css for margin, padding or borders about the images, or about the blocks that contain the images.

If you look at your site in chrome, right click on an image and select "inspect element", the developer window will appear with an analysis of the styles down the right hand side showing how they are applied to the image.

Thanks to both John and beebs93, I really appreciate your reply.

@beebs93... I added display:block to the images as you suggested and it fixed the my issue.

Thanks again for your help.