To compress pictures in Microsoft PowerPoint 2013, you can follow these steps:

1. Open your PowerPoint presentation in Microsoft PowerPoint 2013.
2. Click on the picture that you want to compress. The Picture Tools Format tab will appear in the ribbon at the top.
3. On the Picture Tools Format tab, click on the Compress Pictures option in the Adjust group. A dialog box will appear.
4. In the Compress Pictures dialog box, make sure the “Apply to selected pictures only” checkbox is checked.
5. Choose the resolution or target output you desire for the compressed pictures. You can select from options like “Print”, “Web”, “E-mail”, or “Screen”. Each option has different levels of compression. You can also choose the “Options” button to customize the compression settings further.
6. If you want to delete cropped areas of pictures, check the “Delete cropped areas of pictures” checkbox.
7. Click on the OK button. PowerPoint will compress the selected picture(s) according to the chosen settings.

Please note that compressing pictures can reduce the image quality and may result in a loss of details. Therefore, it’s a good idea to keep a backup of the original pictures before compressing them in case you need to revert to the higher-quality version later.