Okay, you'll still have to play around a little with the image but here is what I found for MS Word.
1. Go to File > Page Set Up
2. On Margins tab, set all margins to 0 and Orientation to Landscape
3. On Paper tab, choose the envelope size you are using (Number 10 is the long envelope) or measure the envelope you are printing on and choose custom and enter your measurements.
4. On Layout tab, under the section "page" choose center for the vertical alignment.
5. Click "Okay" button. A warning window will appear. Click "Ignore"
6. Make sure your alignment is set for "Center"
7. Go to Insert > Picture > From File and locate the envelope you wish to print
8. Click once on picture and resize to envelope if necessary/want. You are now ready to print.