By default MS Word attempts to compress images. To stop it from doing this,
1. Go to File > Options.
2. Click on Advanced. Relevant settings will appear in the Image Size and Quality area.
3. Uncheck Discard editing data, check Do not compress images in file and Set default target output to 220 ppi.
4. Click on the drop-down link bearing the name of the current document (New Microsoft Word Document (4) in the following screenshot). Select All New Documents (or leave at the current setting to apply for the open document only).
5. Click OK.