When creating a broadcast message, you have the option to upload an MMS to your message.
You should find the option to upload an image as an MMS in Step 4 when creating a broadcast.
Make sure that the image file size does not exceed 5MB. Also, check out the supported file types here - https://www.twilio.com/docs/sms/accepted-mime-types
Note - please make sure that you're using a local number, not toll-free numbers as they're not MMS-able.