Thanks for posting on the forum and welcome to the community.
We can understand the concern and confusion caused by the recent charges and as @apcyberax has mentioned you don't actually have to send an image for the messages to be converted from SMS (text) to MMS (picture message)
This can be for a number of reasons as mentioned in the previous post. SMS messages are limited to 160 characters before it converts to two messages, should this exceed 480 it may be converted from 3 texts to an MMS. These characters would include if it was sent to multiple contacts.