What is a Design Mark?
A design mark is used to register a stylized logo, design element, graphic, or image. This could protect your brand name if you plan to use a specific color, font style, or size for your logo. These are referred to as Special Form Drawings on the USPTO website.
Design marks are often much more specific than word marks. A design mark will only protect the exact design that it was filed for. As a consequence of that, even slight changes to a logo over time could warrant filing a new trademark.
For example, if you filed for a colored design mark with a blue logo and later changed it to be red, it would no longer be protected by the design mark. Even if the color change was the only difference, it would no longer be consistent with the original mark.
Also, you have Common Law Trademark Rights in the logo, which you can enforce in federal court if someone were to use it for similar services.
Finally, you likely have Copyright In The Artwork. Therefore, if someone copies the logo, you can assert you copyright against the infringer.