How I made postcards for my blog using Photoshop Elements (apologies to all who use metric):
1. Open three new blank files, the picture you want for the front and the picture you want for the stamp. I make the new blank files 11 by 8 inches.
2. For a simple one image front, drop your chosen photo on top of one of the blank files and resize so that there is a bit of white space to create a frame. If the "postcard" will be on a white background you may want to change the color of the background. Flatten image so it is one layer. Save.
2. To make the back of the postcard select one of the blank files and turn the grid on. "Grid" is found under "View" on the tool bar. Select the pencil icon and choose what width you want your line to be. Draw a vertical line where you want the division to be for the writing and the address. The grid will make it easier to draw the line. After the line is drawn I turn the grid off.
Select the text tool and draw a text box to the left hand side. Write your text.
Select the text tool and draw a text box to the right hand side where you want the address. Fill in.
3. To make a stamp, drop the photo you want on to a blank document. Resize so that the image is quite small, a little over an inch. I use a cookie cutter tool (one of the shape type) to create textured edges and then crop the photo so that it's finished size is about an inch. Flatten image so it is one layer. You can now drop the stamp onto the back of the postcard and position it in the right hand corner.
4. After the text is completed and the stamp has been added. Flatten the image and save. You now have a postcard front and a postcard back.
5. At this time I haven't tried printing the postcards. That will be another adventure...
If you want to put the postcards on your blog be sure to save the photos as a JPEG or some other format that your blog can upload.