@missy1,
To send a text with an attached photograph or sound bite you need to get your text ready to send, tap mobile data on, then send the text, and turn your data off as soon as the transmission is completed.
Don't waste your mobile data by leaving it on all the time.
There are specialty apps that send messages over WiFi instead of the phone lines but both the sender and the receiver have to use the same app
I'm assuming you're sending a typical text which uses phone lines for transmission. In that case having WIFI on or off while sending a text makes no difference.