In order for MMS / Picture Messaging to work, you will need to contact us by email to help@usaprepaidsimcard.com.au (and tell us your USA mobile number) or by lodging a support ticket above.
MMS / Picture Messaging will be activated within 24 hours. MMS / Picture Messages to Australia (or outside the USA) cost 50c (even if you have a plan with unlimited SMS messages). You would need to add extra credit to your account to MMS to Australia.
Alternatively, consider using an app such as Viber or Whatsapp which use data to send pictures and videos. Or if you're using an iPhone, then you can send pictures and video through iMessage.