Picture messages (MMS) will use 3 credits per recipient. If you're sending a picture to a list of 500 people, it will use 1,500 credits.
Picture messages can also include up to 765 standard characters without increasing the credit cost. So if you include a photo and 500 standard characters, it will still use 3 credits.
Note: If you include a special character like an emoji or accented character, the character limit will be 335 characters.