If you want to design your own postcard, there's no need to start from scratch. You can find some great templates on Office.com, which you can access from inside of Microsoft Word.
Here's an example of how to modify a picture postcard template to make it your own.
First, if working with high resolution art, you may want to adjust your settings to retain the original level of detail for best results when printing. In Word 2010, go to File, Options, Advanced and check the box next to Do not compress images in file:
Now go to File, New and type "postcard" in the Office.com Templates search box:
Select your template. I chose the holiday-themed one since it offered a design at the standard 4x6 size and conserves paper by putting two cards on a page.
The first thing you'll want to do is change the picture. It helps to crop your picture ahead of time to the appropriate proportions, though you can adjust it inside of Word if needed. Select the image and then click Insert, Picture:
Right click your image and choose Size and Position (or Format Picture if Size and Position doesn't appear). Check that the height and width are correct. If it's close but not exactly 4x6, you may want to simply uncheck Lock aspect ratio and then enter the absolute values (note that this may visibly warp your picture's appearance if it's not very close to the correct dimensions):
Now modify the text and make any other changes:
Be sure to follow local postal regulations! If you're not sure that your design is legal for mailing, take a sample to the post office for inspection before printing.
If you're printing these yourself, you'll want to do a test run on cheap paper before switching to expensive card stock. Print to both sides of the paper and make sure the fronts of the cards both line up with the backs. Then cut them out (ideally using a paper cutter for precision trimming) to confirm exact layout before final print. You may even want print in small batches, stopping to spot check the consistency of your results.
Alternatively, you could save yourself the hassle and take your documents to a professional printer. You should provide the original Word docs as well as XPS or PDF format exports that contain the embedded fonts, just in case the printer doesn't have the fonts you selected.