Pictures are one of the most important content types you add on your slides — however, each picture you insert may have different resolutions — and thus even though you may have sized your picture to look like a small postage stamp on your slide, it may be increasing your file size by several megabytes. You can of course manually compress pictures in your presentation. Additionally, you can set the document resolution for any presentation — this option will compress pictures you insert automatically to the default resolution you set.
Learn how to set document resolution in PowerPoint 2013 for Windows.

