You do not have to use slideware to make a good presentation. But if you do decide to use PowerPoint or Keynote for visual support, there is no point in doing the same old "usual PowerPoint thing." Be different. Go visual. When "going visual," typically you will use only (high-quality) JPEGs for photographic images in slideware and little or no manipulation of the images is needed. However, if your want to kick your images up a notch or two, trying using images with alpha channel masks.
What's an alpha channel mask?
The Apple site shows a good example of using an alpha channel mask on a photo of a magnifying class to obtain a transparency effect. And Indezine has a very good introduction to alpha channels and Powerpoint.
In Keynote, and now in PowerPoint too, you can include images with alpha channel masks. These images can be quite useful. If, for example, you want the background of an image to be 100% transparent, you can add a mask in Photoshop and "delete" the parts of the image you want to appear transparent, revealing the background of your slide. You can see what I am talking about in the two slides below.
The slide above (left) uses a PNG file with only the subject selected, not the whole image. The slide on the right has the JPEG version without the mask. PowerPoint has a tool that will delete the background color of an image if it is a solid color, but in my experience it does not produce professional results on all images.
I often use multilayered Photoshop files and then save them as PNG files. The downside is that PNG files are very large compared to JPEGs (which do not use masks), but today's laptops are plenty powerful to handle very large PPT/KEY presentations smoothly. Keynote 1.0 handles PNG files, but older versions of PowerPoint do not.
My simple technique
My method is to take an image into Photoshop and then duplicate it by dragging the image to the "Create a new layer" icon on the layers toolbar (see below). Now I have two layers of the image. I then change the bottom layer to a color similar to my slide background. Then on the top layer I can select and delete sections or make selected areas more or less transparent. Then I simply delete the bottom layer, leaving me with just my original image, now with selected areas transparent. You can see the Photoshop "checkered board" underneath the areas that will appear transparent when you place the image in slideware.
Examples of a PNG file over a video in a slide
An interesting, subtle effect you can do quite easily is to have a movie appear behind a transparent part of your image. It is a good idea to use video clips within your slideware, such as an interview with a customer or an expert in the field. Usually when we do this we just have a video clip start, say in it's 320x240 window. But you could place the video within an image of a TV or even make it appear as a reflection of a camera lens. Download these two videos below (exported to QT from Keynote) to see short samples of the slides (each less than 2MB). Both examples feature a single slide with running video (the slide on the right has a bit of animation as well).
Below are screenshots which describe a bit of how I made the slide with a single JPEG (purchased from istockphoto) which I edited and saved as a PNG file and placed over three versions of the same small QuickTime movie.
In the original image (above left) I selected the lens (with 1 pixel feathered for smoothness) and cut it out. I then saved the lens as a separate file. Next (above right) is the image placed in the slide. You can see the slide background were the lens used to be. I then placed the image of the lens back were it should be, but now I can control its opacity separate from the entire larger image (because it is actually a separate image). Alternatively, I could have just selected the lens area and lowered the opacity directly on the larger original image (that's what I did with the eyes of the woman), but then I could not adjust the opacity of the lens directly in the slideware separate from the larger image.
To illustrate the lowered opacity of the lens and eyes in the image, I placed a simple orange box (PPT/KEY object) on top in the slide (above left). When I send the orange object to the back, you can easily see how the larger imaged is masked, revealing what is underneath (above right).
(Above left) I place three copies of the short video clip (of our band in the studio, sans audio) on top of the larger image in the slide. Then I send the videos to the back (right) and the moving images appear as reflections in the lens and the eyes.
The example above is used to show the possibilities. Frankly, I do not know if you would ever want to show reflections of moving images in the iris of the human eye. This is an example of technique only.
Note: You might be saying to yourself that all this work seems to violate the idea of keeping presentations simple. However, whether or not you find this image technique useful, supremely superfluous, or just too complicated is entirely up to you and your unique situation. You do not need to know all the techniques Photoshop has to offer in order to "go visual" with your presentations. However, if you think it is appropriate for your situation, now or sometime in the future, I can promise you that knowing basic Photoshop techniques will prove to be very useful for you.