If you're using 'bundles' (of minutes, texts and data) on your phone, it's normal for normal (SMS) text messages to be included in the deal but not multi-media (MMS) ones, which are used to send pictures. To send MMS messages, you need to
(a) have additional ('non-bundle') credit on your phone ; and
(b) ensure that your phone service provider has enabled the feature on your account.
You might find that you experience a similar problem if you try to send a long text message, as most modern phones automatically switch from using the SMS system (which would require a long message to be split into several separate texts) to using the MMS one (which sends a long message as if it was a picture). Vast numbers of people who use 'bundles' on their phones have been left wondering why their long texts never go through, with the reason being that they've not paid extra for some non-bundle credit in order to be able to do so.
As others have said though, a workaround is to use email or something like Whatsapp to send the message. That then uses data from your allowance, rather than trying to use MMS messaging (which you've not paid for).