Why do I need gigapixels? Can't I just take a regular photograph that I took and enlarge it to mural-size?

Yes, you can - the question is, what will it look like when it's been enlarged that much?

When computer software is used to enlarge digital photographs for printing, it does one of two things - it either makes all of the pixels bigger, or it tries to make more pixels by guessing what color should go in between each pair. That is, if it sees a black pixel and a white pixel, it might try to put a gray pixel in between them when the photo is enlarged. Some softwares do a combination of both.

The problem is that the software isn't yet quite as magical as what you see in TV crime shows.

This is a mural of Athens City Hall. It looks pretty darned good from across the street, doesn't it?

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This is the same mural from about 5 feet away. How does it look now?

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If an impressionist painting is the effect you're going for, then it still looks fine.
If you want something that can be recognized and enjoyed close-up, you need a bigger image to start with.

Now let's look at a Gigapixel image for comparison:

Here's one of my Gigapixel panoramas printed at roughly 30% size. It's 50 inches high by about 16 feet wide.

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And here's a detail of it from about three inches away (the dime is there for scale):

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. . .and that's the difference.

The gigapixel image has full photographic resolution close-up. A standard photo that's enlarged to mural size only looks good from feet or even yards away.

To have really big images that can be enjoyed up-close, you have to start with images that are really, really big.