If you buy stock photo images from a commercial website (see my article), there is a good chance you will be offered a choice of image sizes and resolutions at different price points (not all stock photography sites work this way, but many do). There are two separate measurements associated with the image.
The first measurement is the image size in pixels. You should see two numbers… height and width. A 640x800 image is made up of 640 pixels in the vertical direction and 800 pixels in the horizontal direction. Notice that this does NOT correspond to any specific size in inches. A 640x800 picture would fill the entire screen of a monitor running in 640x800 resolution and would show as a smaller rectangle on a monitor set to 1080x1920. The physical screen size of the monitor determines the physical size of your image when displayed.
The second measurement is DPI. DPI stands for “Dots Per Inch” and relates ONLY to how the image will be printed. In some circles, the term has been updated to PPI for “Pixels Per Inch” – but this is still uncommon on photography sites. High DPI (usually 300 on the stock sites) means the picture prints smaller, with more dots of ink per inch on the paper. This translates to a higher quality printed image. A low DPI (usually 72 or 96 on these sites) makes the image print larger but more “grainy”, since there are larger gaps between the drops of ink.
If you don’t care about the print quality of your images, just ignore the DPI measurement. It has zero effect on the visual quality of the picture on your slide. But for highest visual clarity, buy the largest pixel size picture you can afford. You can always shrink down the graphic to make it smaller on the screen. It will still look swell. But if you go cheap and buy a small graphic and then try to expand it to fill your slide, you’ll end up with a fuzzy picture. Your goal is to never expand a graphic image from its original imported size. You always want to reduce the size on the slide to suit your needs.
When you purchase the image, save it to a directory on your hard drive. Then use the “Insert Picture” command in PowerPoint to import the picture from the hard drive using its file name. Avoid the temptation to copy and paste images onto PowerPoint slides. Copy and paste in PowerPoint results in lower quality and higher PowerPoint file sizes.
If you need to change the size of the picture on your slide, always grab the resizing frame by a corner, never a side. Resizing from a corner retains the aspect ratio of the picture so things don’t get stretched or smooshed unnaturally. Don’t worry about the picture being too large in one of the dimensions at this point… You can crop it to fit the space available.
Here’s another tip. If you want your picture to fill the slide, start by sliding it to get one of the corners aligned with the corner of the slide. It will snap into place as it lines up. Then as you resize the picture (using the corner!) it will snap into place as you get to the opposite side of the slide. Pictures almost never match up with your slide size in both dimensions. Don’t worry… Just expand it to cover the full slide and let the image extend past the edge that doesn’t fit.
Once you have the general size where you want it, use the Format-Crop command in PowerPoint to remove the unwanted extra parts of the image. This is an important step in reducing file size. But you need to add one more step. Once your presentation is assembled and finalized so that you are happy with all the graphics, click on any picture on any slide and choose Format-Compress Pictures. This brings up a dialog box. Make sure the command applies to all pictures in the file and choose “Delete cropped areas of pictures.” This eliminates the cropped bits and cuts down the file size. If you leave out this step, PowerPoint retains the cropped information just in case you ever change your mind and want to uncrop the image.
By the way, the compression command also lets you reduce the DPI of images on your slides. This is another great way to reduce file size. But be careful… You can reduce DPI to the point where slides look ugly when printed! You may want to save the file in two forms… One for distribution and printing, and another for use on screen and for emailing to others for review. Compress the heck out of the second version, safe in the knowledge that you have a higher resolution version still available if needed.