As a designer for both print and web, I run into trouble when I try to explain why I need high resolution photos for print design and lower resolution photos for websites. Here's to hoping this analogy makes it just a little easier to understand:
Imagine you work for a little corner pizza place. You are the one in charge of rolling out the pizza dough, throwing on the toppings and getting the pizzas into the oven. Your restaurant sells two kinds of pizzas: Personal and Extra Large, and you are really good at making both.
One day, a coworker says, "We've got a big group coming in this afternoon. Make sure you make enough pizza to satisfy the hunger of ten people." He then hands you a bowl with just a small chunk of dough.
"This isn't going to be enough dough," you say. "This might work for a personal size pizza, but definitely won't work for feeding 10 people."
"Can't you just stretch the dough out?" he responds. "I've seen cooks on the Food Network do that."
"That's fake," you respond. "It would be much simpler, and the pizza would be much better, if you gave me enough dough to make an Extra Large pizza."
"Oh, okay," and then your coworker goes and makes enough dough for an Extra Large pizza, and everyone gets enough to eat.
The "Extra Large Pizza" in this case is comparable to a design project for print. Professional printers need large photos to make those graphics look great. A little tiny photo that would look good on a business card can not be blown up to look great at 21 inches tall (no matter what special effects you see on TV).
Let's look at another pizza conundrum.
A teenager walks into your restaurant and wants a pepperoni personal pizza. Your coworker says he'll handle this one, and your busy, so you let him take the reigns. You watch from across the room as he takes a huge clump of pizza dough and smashes it onto the prep table. It's enough dough for an Extra Large pizza, but he forces into into a mound of dough roughly the same size as a personal pizza (and 4 inches tall in the middle).
"You probably shouldn't serve that," you say as you walk over. "That's way too much dough for a little personal size pizza."
"It looks like the same size as a personal size pizza," he responds. "What's the big deal?"
"While it may look like a smaller pizza, you are giving them enough dough for 10 people, and that's just too much."
"So what should I do?" he asks dejectedly.
"Well, it's simple. Just take a small piece of that big lump, and make that into a personal pizza. Problem solved."
The personal pizza is comparable to photos for the web. While printers need big, high resolution photos, websites definitely do not. You want to deliver much smaller photos (less dough) so that your visitors can "digest" those photos easier.
Some website can trick you by allowing you to resize the photo in your page editor. In this case, your making the photo look smaller, but your visitor is still being forced to download all of that information. All of the websites we build come with the ability to automatically scale down the resolution of your photos when they upload, taking a 5 megabyte picture from your digital camera or smart phone, and reducing it down to 100 kilobytes. This can also be done in programs like Photoshop, Photoshop Elements, GIMP and Picasa (to name a few).
Lastly, lets talk about those kilobytes and megabytes. These are just terms to describe the size of a file on your computer. We can compare kilobytes to "feet" in distance measurement, and megabytes to "miles" in distance measurement. 100 feet is definitely shorter than 1 mile. 100 kilobytes is much smaller than 1 megabyte. In fact, it takes 1,024 kilobytes to equal one megabyte. Most digital cameras take photos that are at least 3 megabytes in size. 3+ megabytes is excellent for print, but not so good for the web, where 300 kilobytes or less will more effectively get the job done.
So when serving pizza, make sure to use the right amount of dough. When providing photos for projects, make sure to provide the right image size for the project.