How allow only text in iMessage but not pictures or videos? IOS 9.0.2
I am sure that this is not possible with an iPod Touch. It's not even possible on iOS so unless your carrier allows blocking of MMS and you forgot iMessage entirely, you're not able to control the feed into messages app.

If an iPhone were in play, you could disable iMessage and MMS so allow only SMS text messages, but again, that would require a cellular connection.