I've never used it, but I think this might be what you need.

Genuine Fractals claims to do what you want, but I've never used it, so YMMV.

This seems to be a good tool that can image resize.
From the Slashdot article on the same, there are links to Gimp plugins.

Good Luck.

THANKS, all. I jumped on Rasterbator first, but it wants me to choose the enlarged size in units of millimeters, and I don't know how to convert pixels/mm (and it really depends on DPI anyway, doesn't it?) Aargh. I'll fake it, then try the other two. Thanks again!

Thanks again. I found the original, but I know I'll need to interpolate in the future (soon.) I just can't keep track of my photo files.

Just to fill you in, the Rasterbator is definitely not what you're looking for. It's for making giant posers out of images like this or this. (those are two of mine)

giants posters

I was all excited for a minute there. Okay, then, how do I make a giant poser?

Regular reading of Pitchfork? [I kid.]

matildaben wins ... er ...

Ouch. You can get a larger version, but keep in mind that JPEG compression is not "lossless" -- you do lose data in the compression process. Any expansion is just going to create a "blur" between the two values of adjacent pixels that now need in-between pixels.

The quality you get with this may not be, um, competition-ready.

Generally speaking, enlarging a photo is like trying to put the toothpaste back in the tube. Once a photo is reduced, the information contained at the higher resolution is lost forever. In order to enlarge it again, you have to essentially fake the missing data by guessing what it might have been. This is obviously not a terribly effective strategy, and so the best you can hope for is blurryness and a general loss of detail. You can't just construct information from nothing once it's been discarded. All that crap you see on 24 where they can just click on "enhance" and suddenly see a license plate is total bullshit.

What you seek to do is (at the moment) totally commercially impossible. I have been working with and creating digital art since 1988. The problem you face is an ancient one, and yet unsolved. Basically, you're screwed.

I have used Genuine Fractals. It will not do what you want it to do. It does do a fair job of expanding crappy small images... for advertisng print use only, not for photography. They use it for billboards and shit, where it's miles away and you can't even see the big, huge, freaky looking blocks of fractally interpolation it leaves behind. It's fine if you want a heavily edited poster. Not for photos.

Do it to a fine art photo and it'll look like you ran it though Photoshop's "Watercolor" plugin. Which is unacceptable for pure photo work.

as an addendum, hope your saving in something other than jpg

Well, a Graphic Arts textbook I have says enlarging/interpolation is practical, although you DO lose resolution. Any interpolation program is supposed to fill in those cracks between the pixels in such a way that hopefully helps you out. But maybe they're talking about a professional program that I'll likely never have access to.