How can I expand a photo?

If you don't have a higher resolution image to begin with, you can't add more data to it.

you could resize the image, but you'd get a degradation in image quality.

What's the current res of the image?

D

if it's low res and you blow it up it's not going to look good.

it's easy using iPhoto, though as others mentioned, image quality will degrade.

select the picture in iphoto, File->Export.
choose file export and set the resolution to whatever the values you'd like...

(it says "Scale the photo no larger than..." but it can expand as well as shrink any image.)

Agreed, it'll get blown up, but it's gonna be ugly. You can go down but cannot go up the grade.

Just to let you know along with what everyone else has told you, the image data cannot be expanded because it didn't exist in the first place. Digitally, the pixel size is already set. So when enlarging the photo size the pixels are added but the reason it will look poor is because the computer is "making up" the new pixels.

One thing that might help (maybe) is printing out the photo at its current resolution, then scanning the print at a very high resolution, atleast 500 dpi. It's a method i used once and it worked as much as I needed it to.