Dana has asked the question that drives so many people bananas when it comes to dealing with digital photographs. She's wondering about the size of a digital image, or "How would I determine the DPI for a particular photo I have?"
This question is such a constant problem for so many people, I'm going to settle it once and for all, in the simplest way possible.
The biggest part of the problem is that most people asking the question don't know the correct terminology, and the people answering actually don't have a firm grasp on the situation either, and so just muddy the waters. In this case, the problem is the term DPI.
The thing is, a picture file can be any DPI you want to make it. It's not a fixed property of the file. You change the DPI of the image according to the device on which you wish to print it. What you really need to know is the size of the image in pixels, what device you wish to print on, and a basic math calculation.
The situation Dana is in is this, I assume: she has a digital file of a photo, and she wants to know how to figure out how big it would print if she used it on the cover of her book. Here's the straight answer, using no additional software.
First, Dana, open the folder the picture file is in. (Easiest if it's already on the desktop.)
Next, drag the icon of the image file into your Internet browser window. You heard that right, trust me.
When the photo opens in the browser window, right-click on it and select Properties from the context menu. In the resulting Properties window that opens, you will see an entry for dimensions or height and width (depends on the browser). The size will be listed in pixels - something like 700x 400 for a small image, or 2100x3600 for a big one.
Once you know the size of the image in pixels, divide each number by 300. The resulting 2 numbers give you the dimensions in inches the image will print in high quality colour on a printing press. That's it.
So in our first example above, a 700x400 pixel image will result in a 2.33 inch by 1.33 inch printed photo. In the 2100x3600 pixel example, you'll end up with a 7 by 12 inch photo.
Now obviously, this process doesn't tell you how to prepare the image to actually print at those sizes, it just tells you what's possible. It's a fast rule-of-thumb way to tell if your file is suitable to print at the size you need.
If your calculations show that your file is almost big enough, there are a number of ways to massage it to slightly increase the size it can print, but in general, you want these calculations to tell you that your image can print at least as big as required in it's original state for highest quality. (There's no problem if your file is bigger than necessary - quality isn't lost making an image print smaller.)
And if you want to print on something other than a colour press? For a laser printer, do the same as for a colour press. For an inkjet printer, divide by 240 instead of 300. For black & white reproduction in a newspaper, divide by 150. Yes these are generalisations, but they'll serve for nearly all typical situations.