With HTML, you can style your emails to include pictures and layouts instead of sending only plain text messages. However, Gmail and other email clients do not display HTML in the same way as Web browsers. In particular, Gmail displays white spaces for images by default. Additionally, when pictures are displayed, a few pixels of white space often appear around the border.
When a user receives your HTML emails in Gmail's Web client, none of the images is initially displayed. The email recipient must first choose to show the images. This prevents readers from seeing pornographic or otherwise inappropriate images unexpectedly. The reader can check who sent the email and then decide whether they trust that the email's sender used appropriate pictures. Gmail users can choose to always display images from trusted senders. As the person sending the HTML email, make sure that you do not put all the important email content in images. Use enough plain text for your intended recipients to know what the email is about and decide whether to view your images.
Gmail does not use full cascading style sheets in HTML emails. It ignores any "<style>" tags in the HTML headers. If this is where you put your HTML layout options, your email recipients might see white space. Use tables and inline style to format your HTML emails. Additionally, Gmail often adds a few pixels of white space around the borders of images in HTML emails. You can get around this by using inline styles. Add "style="display: block;" to the end of your image tags to fix this problem.
Your Gmail recipients might see other white space problems with your HTML emails, depending on the client you use to compose the email. For example, some versions of Microsoft Outlook double-space between every line of text in an HTML email by default as it inserts the paragraph tag instead of a line break. Different email composing tools have different issues. Whatever program you use, make sure that you check how your HTML message looks in your own Gmail account before sending it out.
Make sure that any images you have used in the HTML email are uploaded and available on the Internet. Gmail does not display images sent as attachments -- only those that it can retrieve online. Finally, make sure that you include a plain text version of your HTML email for those recipients who have HTML email turned off or who might be reading their Gmail account from an email client that does not have an HTML option.