What size images are you starting with and how large do you want to blow them up.
In general, you can do this somewhat, but as someone said, if there’s not much detail to begin with, Photoshop or any other program will be hard-pressed to add more detail.
One method that people use is to increase in steps of say 10% using Bicubic Smoother. Another way is to use a third-party plugin that is specially made to increase images: Blowup from Alien Skin and Genuine Fractals from onOne software are both good and have trial downloads.
I recently read a review of image enlargement software and both of these rated well. I believe that BlowUp has a few more features, like adding noise which is supposed to help with enlarging some images. See: <
http://www.photoshopsupport.com/tutorials/cb/noise.html>
Some people believe that the Photoshop step method is as good as a third-party plugin, but I found that these plug-ins do a better job if you have to enlarge a great deal. I had to make murals of some digital photos, and used one of these to blow up the iamges by over 300%. (I also have tried Bearded Frog and Kneson software but found them both to be a bit prolematic IMO).
Best, David