You have to use an inkjet printer -- not a laser printer. The ink of the inkjet will absorb into the fabric. Printed Treasures is a great one to use, though EQ also makes a good paper as well.
When you print, if you want vibrant colors, you need to over-saturate the color in the picture before printing (using a picture processing program). Some of the ink is absorbed and if you don't over-saturate the pictures are softer.
I have made a bunch of quilts using pictures over the years using a variety of methods and I have had better success with the commercial printer sheets, the ones I made myself did not hold the color as well even though both quilts were started out as bright and were in basically the same place for five years. I made my first picture quilt 10 years ago, and the fabrics are sooooo much better now.