This video shows you the steps to crop images and export them in file size that are suitable for web use.

This is the steps in textual format:

1. Create a blank presentation.
2. Go to the Design tab > Slide Size button > Custom Slide Size. Key in your desired dimension.
3. You can type the width and height in pixels and PPT will automatically convert it into inches. Click on "Ok".
4. Choose "Maximize Fit" if you are prompted.
5. Place your image on the canvas. You can crop if viewing it cropped is easier for you, otherwise, leaving those extra lengths out of the canvas will automatically be cropped off when you export.
6. To export, go to File > Export > Change File Type and choose "JPEG/PNG" and save as.
7. Choose the location to save and VIOLA!

Hope this tip will help you with your web image production and editing!

Cheers to greater productivity!