If you mean the PC program (rather than the Picasa website which works like Flickr) - select the photos, then choose 'Export picture to folder' from the File menu. Set to resize to 690px, choose where you want them to go, then 'Export'. You can then upload these versions to your reports
The recommended method is still uploading them to another website such as Flickr, Photobucket (or Picasaweb - I presume they let you do this as well) and just linking them to Walkhighlands, rather than uploading them. This is because the other sites will distribute your photos over several different servers which makes them appear in Walk reports quicker.