MMS (picture messages) will use 3 credits per recipient. If sending a picture to a List of 500 people, it will use 1500 credits.​
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Picture messages can also include up to 765 standard characters. Regardless of how many characters are included, a picture message will always use 3 credits per recipient.

Something to note: when using a special character, like an emoji, the character limit will be 335 characters.