Building an overly fancy HTML mail for a client. Code validates at http://validator.w3.org as XHTML 1.0 Transitional by direct input. Thing is Gmail displays gaps between
The specific answer to your question is that Gmail adds extra space to table cells which only contain an image. To fix this issue add to these images:
style="display:block"
Tip: Campaign Monitor is a great resource, as is MailChimp. Both provide several guides, template examples, etc.