If you are not a PhotoShop artist, and use PowerPoint to take out the background of an image instead, the edges of the remaining edges can look a bit ruffled. Sometimes applying an interior shadow to the image solves this problem. Sometimes, because it works brilliantly for some images, and not at all for others. You have to try.
To apply an interior shadow in PowerPoint 2007: click the image, go to format/shape effects/shadows/inner and pick the middle one of the options.