Gaps between images can occur in your emails within the editor and in email clients, especially when the images are separated and put back together using tables. Normally, this issue can be solved by a simple amendment to the HTML code of the images. The code requires a "display:block" style attribute be added to all <img> tags. The code will look like:
<img style="display:block;" height="40px" width="40px" src="imageurl.com">
Adding this addition to the <img> tags within the HTML should solve the issue of gaps appearing underneath and around your images.