Reducing the file size of a photo can be done on both a Mac and on Windows.  The steps are as follows:

 On a Mac (using Preview):

1. Open the image in Preview
2. Go to Tools > Adjust Size
3. Reduce the resolution or dimensions until the file size is under 8MB

On Windows (using Office Picture Manager):

1. Open the image and click Edit Pictures
2. On the right side, choose Compress Pictures
3. Select Web Pages or another preset to reduce the file size