Perhaps the most basic skill to master in PowerPoint is that of inserting pictures. In PowerPoint 2007 and 2010 they've moved to the ribbon menu system so you have different tabs for each of the major features you might need to use. Getting your pictures into PowerPoint uses the "Insert" tab.
I'm just going to say that you should use the "Picture" button here most often. If I see your mouse stray over to that "Clip Art" button I'm going to be very disappointed. Sure there may be one or two times a year that clip art will be appropriate (I'm not kidding that little), but it should never be a regularly used tool for good presentations.
The next thing you'll need to use is the "Picture Tools" tab. This will only show up when you have a picture selected. So click on the picture you just inserted then look around. Over on the right are the first things you need to look at. Adjust the size and crop of the image. The size of a PowerPoint slide is 7.5" high by 10" wide. Most of your pictures should fill the entire canvas. But don't skew your image to get it to fit. It's better to make your image larger than the slide then crop off any excess bits so the picture doesn't looked stretched and funny. A quick way to make sure this doesn't happen is to click on the dot at the corner of the picture and drag to resize the image while holding down the "Shift" key on your keyboard. The image will keep the same height-to-width ratio as the original.
Crop the excess by clicking that crop button. Now the dots at the corners of the picture have turned into black brackets. Drag the brackets in or out to crop the picture to that size. This can be a quick way to edit photos without having to open up a separate photo editor. Click anywhere off of the image to exit the cropping tool. You can undo the cropping if you want, the rest of the image is still there until you close PowerPoint, so you can play with different views to see what works best.
Play with some of the different Format tools that you can use to change the appearance of pictures. What do you like? What tips do you have for inserting pictures into PowerPoint?