I reduced the image size of some photos out of ...

You certainly can "upsample" a definition of, e.g. 640x427 [to a higher definition] using an image manipulation app such as Photoshop, but you'll lose *resolution* the higher you go....

Go ahead and use your original full definition files, e.g. 2048x(xxxx) to carry out any hard copy print operations you wish to perform....

When reducing the resolution of an image, the program basically analize the color date of each pixel and get rid of a lot of pixel in order to make the image smaller. it doesn't keep the information about the lost pixels anywhere. So there really isn't a way to go back to what you had.

You can, however, increase the size and resolution (number of pixels per inch) it won't give you a result that's as good as the original, but it will be larger.

The best way to do it is, instead of  increasing the image to the size you want it to be all at once, is to increase it size to 110% (on image size) and then repeat the process till the image is the size you want it to be, or until it start to get blurry or pixelate (whatever comes first).

It depends on the image and it's size, but in some images I manage to do  that 6 or 7 times and the result was pretty good.

lilly, it's not recommended to do several manipulations.

i recommend you to resize it once to 200% and then to retouch the image (do it gently :)  ) 

I often have to resize upwards a pic or drawing I have done and for this i use the best program available and that is IRFANVIEW...none bettter for this type of operation.

It can be found here  http://www.irfanview.com/

This is one of the best pic manipulation programs that is available and is a must to have. It does so many things and above all it is FREE FREE FREE

Give it  a try

Regards Rick