My main concerns were addressed by both PrintDriver and kemingMatters. I completely agree with both of them.
The side of the postcard where stamps, addresses, writing and various postal indicia needs to go doesn’t leave much room for imagery. If you look at most any postcard, you’ll find this being the case. It’s a purely practical matter, and as PrintDriver mentioned, form always follows function — first and foremost, a postcard needs to function as a postcard.
As for the colors of the postcard and, for that matter, the imagery itself, I’m unsure why you think they’re quintessentially Australian. These photos of wetlands could be most anywhere in the world, and if I had to guess where they were shot, Australia probably wouldn’t come to mind.
I’m certain parts of the country look like this, but what I’m saying is that if the assignment is to use the “colours of Australia,” those colors (and the imagery) should be unmistakably Australian.
So what are the colors of Australia? From my point of view as an American, when I conjure up images of Australia, I think of the deep blues of the ocean and the sky, and I think of the earthy reds of the outback.
Similarly, when I think of Australian images, I think of Sydney Harbour, Ayers Rock, various marsupials, The Great Barrier Reef, etc. Maybe you don’t want your postcard to represent those stereotypically Australian images, which is totally fine, but you don’t want your photos to look like they could have been shot in Saskatchewan or the Pampas of Argentina either.
I’d suggest doing a little research. For example, a Google image search for Australian postcards will give you a good representative sampling of what other designers have come up with (both good and bad) when faced with similar assignments.
One more thing: it’s generally not advisable to add gratuitous drop shadows, glows, bevels and other Photoshop filter treatments to typography, like you’ve done on the “Colours of Australia” headline. I’m not saying these filters should never be used, but an over-reliance on them to add pizzazz to type is a sure sign of a beginner using one-off gimmicks instead of good, cohesive and well-thought-out design decisions.
Despite all this criticism, please don’t be discouraged. As you said, you’re not a designer (yet). I actually have the first design class assignment I ever completed hanging on the wall in my home office. It’s absolutely terrible, but I keep it there as a reminder to myself of my first less-than-solid steps on what has turned out to be a very long journey.