Out The Back Australia Webmaster - Awesome website! I've only traveled outside of the United States of America one time and that was to the East Coast of Australia. It was a great experience and I'm kind of under the impression that the "arrive a stranger and leave as a friend" (in About Us page) is typical anywhere in Australia. I hope to visit again someday! ...Now, focusing on your website... :-)
Joe pointed out a serious issue with the graphics; Sometimes browsers fail to use the best discretion when resizing an over sized image - so on occasion I've found bigger images fail to even look as sharp as a properly sized smaller image.
If you haven't, you might want to check out Lee's tutorial on working with images here. DNN makes it super easy to resize images from the image manager, select an image file and you'll see the image preview to the right. Select the image editor link/button above the image preview. After the image editor opens, on the right side you'll see adjustments for opacity width and height - make sure the check-box beneath these is selected to constrain x/y proportions then just adjust the slider or up/down arrows to the dimensions nearest what you require (ought to be able to type the size you want in the text-boxes but I've never been able to, maybe in a future release?).
Navigation links:
Good choice of skin/template from Artisteer. It looks good but the top horizontal navigation wasn't defined to hold as many links as you're using. This looks fine in IE 8 and Firefox but in Chrome, Safari and earlier versions of IE the links overlap the top of your module containers t itle areas. For a quick glance at your site in different browsers, try something like: http://browsershots.org
One way to prevent the overlap is by editing your portal skins skin.css file [of course don't edit or change any files without first backing everything up!!], open skin.css and search for ".art-nav", look for this with {z-index:100; position:relative;} (should be line 520) add "height:51px;" A MUCH better choice though would be to consolidate and simplify your navigation so everything fits on one line. Gallery and Media should be able to go together, Contact Us could be a link under About Us (also a link in your footer?).
The "Arial Black" font has a slick look but I wouldn't recommend using it at any less than 18px or about 1.4em else it starts loosing its smooth curves and letters start running into each other. That's defined here: http://www.outthebackaustralia.com.au/Portals/_default/Skins/OTBAPgetmp/style.css in the style definition ".art-menu a .t" If you go there and try removing the "Arial Black" so that Arial is first, change the font size from 12 to 15px and font weight from normal to bold... I think you'll find it makes your navigation a lot cleaner and easier to read.
I'd suggest also looking into modifying the drop-down menu background color to set it apart from the rest of the page. The skin you're using uses a simple image file to style this either white or yellow depending on if the mouse is over the link. If you'd like to try changing the color, edit this file: http://www.outthebackaustralia.com.au/Portals/_default/Skins/OTBAPgetmp/images/subitem.png - change the white pixels to some other fitting color (really light yellow/orange like #ffeebb perhaps?) that helps the navigation stay above the page contents rather than tend to get muddled in with it.
Yslow is a fantastic tool (the one Joe mentioned for testing your site/image weights)! You need Firefox with the Firebug add-on first in order to use it... If you don't have this, get it!
You mention on your front page a Thank you for visiting the site and that the page is still under construction - generally people are skimming fast and these sorts of messages don't add much value... especially the "under construction" - Just as growing/developing cities are always in the midst of various construction projects, so should your always improving website(s).
You're web site is well on the way, great work!