Steps
Start with a package of T-shirt transfer paper, available at office supply places.  Print your favourite photos onto the paper.  Use the 'Mirror Image' setting to reverse the printed image so that the ironed-on image won't be backwards.

Cut a blank from plywood (TIP: Use Baltic Birch plywood - it's the nicest and you can buy it in small 2'x2' sheets at the Home Depot).  Clamp a straight edge (like a carpenter's square or another piece of lumber) so the saw cuts a straight line.  If you don't like power tools, you can easily cut Baltic Birch with a handsaw, or cut curvy shapes with a jigsaw.

Sand the surface of the wood.  The smoother it is, the better it will accept the transfer.  If you sand starting with 100-grit and move up gradually to 180 or 220 grit sandpaper, you'll have a really nice transfer surface.  On the other hand, if you want a more antique, flaking look, leave the surface of the wood unsanded.

After sanding, trim the white edges around your photo images (I like using a paper trimmer, but scissors work fine) and place the transfer paper image-side-down on the wood.

The iron has to be used dry (no steam!) on a fairly high setting to get the plastic film on the transfer paper to release.  Press down and move the iron over the image for about 2 minutes.  change directions and maintain steady pressure, making sure to evenly heat the entire surface of the paper.

Test a corner of the heated transfer sheet after about two minutes to see if it lifts easily.  If not, apply heat slowly and evenly to the edge.  Then peel away slowly, keeping the iron on the rest of the paper to maintain the heat.

Keep peeling, slowly and steadily.  If some of the plastic/image lifts, don't sweat it.  You can patch it later.  Or just leave it rustic.

If you are placing several images side by side like I did when I made my office chair mat, you'll end up with some space between 'seams'.  You can patch seams and bald areas by cutting a similarly-shaped scrap from unused images, and applying it to the naked spot.

Iron the patch in place using a sheet of parchment paper (the stuff you line cookie sheets with), which will prevent the iron from tearing up the already transferred image around the patch.

Now the bald spot is gone.  If you have the opposite problem - dark overlapping seams - you can apply a similar solution.  Place parchment paper over the seam and place the iron on the seam for about 30 seconds.  This will heat up the seam. Then use the tip of your iron through the parchment paper to nudge/melt the seams together.

Seams can be minimized on large pieces but they're still visible.  I actually like the vintage linoleum effect of the seams.

This is the chair mat.  I took photos of the rug that's already in the office and transferred them to the mat.  Now I have a nice chair mat that doesn't scream plywood.  (The reason I need a wooden chair mat instead of a plastic see-through one, is that the floor is so un-level that I have to shim the mat up about an inch and a half on one side so I can be sitting on a horizontal surface.  You can't shim a plastic mat.)

This is how the chair mat looks under the desk I made from bamboo plywood.

Have fun with your photo transfer projects and send me photos if you do some cool stuff.

(Photo right) Lap desks (which double as TV trays), a mousepad and coasters (or wall tiles!) and (left) a veneer votive holder.