 The most common problem encountered with creating
PowerPoint presentations is …
File size
PowerPoint file size become so large that it becomes problematic to move around.

The reason for this is the choice of image resolution, and image dimensions.
In PowerPoint presentations the image is displayed on the screen ….
But before we continue we must be take a little detour …

What is meant by “resolution”?
For digital images resolution means the number of pixels per inch (dots per inch, dpi). Typically printers need images to be
300 or 600 dpi.

For computer displays screen resolution is the total number of dots (pixels) on the entire screen. For example, a 640 by 480 pixel screen is capable of displaying 640 distinct dots on each of 480 lines
(about 300,000 pixels). This translates into different dpi measurements depending on the size of the screen. For example, a 15-inch VGA monitor (640*480) displays about 50 dots per inch.
http://www.webopedia.com/TERM/r/resolution.html
End of detour

If we scan an image at 50 dpi and rescan it at 300 dpi the second image will be 36 times bigger (it is 6 times wider AND 6 times taller). Different image file types
(jpeg, bmp, gif) use different compression techniques so file sizes of images stored on the hard disk may vary considerably.

In this screen capture the second column indicates the size of the (compressed) jpeg file as it is stored on the disk. The column on the right was obtained by opening each file in Photoshop and represents the uncompressed size of the image.

If a computer screen is capable of displaying at 50 dpi only there is no advantage in scanning the image at 300 dpi. The image is imbedded in the PowerPoint file.

The other factor to consider is the display size of the image in
PowerPoint. If we scan an image that is (say) 20cm by 30cm but want it to occupy (say) 10cm by 15cm of the screen real estate we could decrease its file size by a factor of 4 by
its canvas size to 10cm by 15cm.

Both centa300.jpg and centa300a.jpg have an image resolution of 300 dpi. The images centa300.jpg and centa300a.jpg have canvas sizes of 274.49*171.2 mm and 137.24*85.6 mm respectively.
Halving the canvas size dimensions roughly (not all of the image file is dedicated to the image pixels) quarters the file size.

This is image centa72a.jpg which has a canvas size of 137.23*85.73 mm i.e. similar to conta300a.jpg (see an earlier slide)

I would recommend scanning at 100dpi as this allows a degree of flexibility if you decide you want to increase the display size on your PowerPoint slide. This allows you to increase the display size by
(typically) 50% before pixelation becomes a problem.
Left Image :- scanned at 72 dpi
Right Image :- scanned at 30 dpi, resized in PowerPoint to be the same size as Left
Image.
Note - Right Image is “pixelated”

The above segment of a screen capture shows the file sizes (jpeg format) of a picture scanned at 72 dpi, 300 dpi and
600 dpi. As each of these files represents a separate scan there is a slight discrepancy in the image dimensions.
centa72a.jpg = 137.23*85.73 mm centa300a.jpg = 137.24*85.6 mm centa600.jpg = 137.16*85.56 mm
(close enough for this demonstration)

This segment shows the PowerPoint file sizes resulting from a single page with the same background as this slide and with a single image inserted.
These PowerPoint presentations are also available for you to view to convince yourselves that there is no real difference in display quality but a real difference in file size.

Now let us insert a few images created using the above guidelines (actually scanning at 72 dpi and with a canvas size of ~137*85 mm) ….

Although this PowerPoint presentation has a modest number of images inserted in it, it is still quite a manageable size.

Aside:
If you wish to scan for inclusion in a printable document then the above discussion still holds but the scan resolution is 300 dpi. If you also want to use the scanned image in a PowerPoint presentation then consider scanning at
300 dpi (for printing) and using
Photoshop to resize the image to 72 dpi for PowerPoint. Don’t forget to use different filenames for the different resolutions.