Sending images with text messages does not always work

I often need to send images along with text message. Sometimes they will send, sometimes they won't. For example. The other day, a contractor working for me was at a merchant purchasing supplies. He called me and asked me to send image data to the merchant. He gave me merchant's cell phone number, but the images will not transfer. He then had the merchant text me. I was able to send then images in a reply message to the merchant. I have had a similar incident when I tried to send images in reply to a text message, but they won't send. Why does this work sometimes and not others?

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