An app I like for my iPad is Haiku Deck. What is nice about Haiku deck is that it makes creating a presentation not only easy, but beautiful. The hangup is that Haiku deck is only available for the iPad.
Haiku Deck automatically searches for images using the keywords in your title slide. The images are high quality and dynamic. The first trick to replicating this is to find high quality images.
I stick to Creative Commons images. One place to find Creative Commons images is on Flickr. Rather than going directly to Flickr, I use http://compfight.com. I can not only find creative commons images from Flickr on Compfight, but it gives me the attribution information that I can copy and paste.
Go to http://compfight.com and search for a keyword.
Click on the image you want to use in your slides.
Paste the photo credit into the slide notes or create a text box off canvas and paste the photo credit in there. It is suggested to create an attribution slide at the end of your presentation where you paste and organize the credits. You can remove the tags from the credit.
Photo Credit: http://www.flickr.com/photos/31191642@N05/3932052676/ by Sergiu Bacioiu via http://compfight.com, Compfight. Creative Commons licence http://creativecommons.org/licenses/by-nc/2.0/
This gets a little easier with subsequent text boxes. The custom color is saved in the custom color tray. Instead of choosing a white background and then going back and customizing the translucency, simply choose the first custom color box. Notice that it does not look like anything is there since your custom color is white. But trust me, it works.