As of 2015, images of various types of tumors in dogs are available on the veterinary medicine section of About.com. The Long Beach Animal Hospital website devotes a page to images and information on mast cell tumors in dogs.
Mast cell tumors are the most common type of tumor that affects dogs, with about 20 percent of dogs developing a malignant mast cell tumor at some point in their lives, according to Long Beach Animal Hospital. There are two categories of mast cell tumors: cutaneous and systemic. Cutaneous mast cell tumors are more common and form on the skin, while systemic tumors affect the internal organs, most often the spleen, liver or lymph nodes. Benign mast cell tumors tend to have well-defined margins and grow slowly, while malignant tumors have poorly defined margins and grow at a faster rate. Benign variations of this type of tumor also do not usually exhibit much inflammation, while malignant tumors are often painful and ulcerated.
Another common form of skin tumor in dogs is the basal cell tumor, notes PetCareRx. These tumors usually take the form of single, hairless bumps. They are usually benign and do not spread very often, so surgical removal typically eliminates any problems they cause. Common places for dogs to develop basal cell tumors include on the neck, head and shoulders.