I’m guessing that a pretty big percentage of images used in PowerPoint come from the web. And I’m also guessing that most people may realize that most images on the web have been compressed in some way to provide a balance of quality & small file size. What I don’t think many people realize is that when you copy & paste images directly from the web into PowerPoint that you lose much of that compression and wind up with a bigger file size than you need to have.
The reason is that whenever a picture is copied to the computer’s clipboard and then pasted into PowerPoint (or Word, etc) the picture is pasted as a bitmap—regardless of the file format of the original picture. Bitmap file sizes are much larger and have less detail which leaves you with a much larger Word or PowerPoint file than necessary, particularly if you have used multiple images this way.
So instead of copying & pasting directly from the web into your document, you should save them first and then Insert them via the Insert menu instead.
I did a quick test with the following results:
File Size (kb)
|Copy & Paste||
|Copy & Paste (Compressed)||
Notice that even after using PowerPoint’s option for compressing the pasted images the file size was still larger than the file using inserted images.