Understanding Resolution

How color works (RGB)
All televisions and computer screens create an illusion of many colors through a combination of only three: Red, Green and Blue. By the principle of the "additive color theory", equal combinations of red, green, and blue light make white light. This system works because it is how our eyes work--our eyes discern all the colors we see
by using "cones" that are sensitive to red, green, and blue light. By mixing combinations of these three colors, we can fool our eyes into seeing yellow, for instance, when it is really equal parts red and green light.

Early digital cameras were only black and white, but could reproduce color if the photographer shot three separate images with red, green, and blue filters on the front of the lens. Then, each of them would be colored in the computer respectively and combined to form a full-color image. This is really how it is still done, but newer, more sophisticated cameras can capture all three colors in the same instant. Digital images are composed of separate "channels" of Red, Green, and Blue.

Resolution
Digital images have a resolution (pixels/inch). There is always a finite number of pixels in any digital image. Generally, the more pixels that are in the image, the more "details" that can be described or rendered by these pixels. In a mosaic, the achievement of realism is limited by the size and number of the tiles. There is of course a trend in digital cameras toward greater and greater numbers of pixels per image; because the number of pixels is proprtionate to the detail of the image.

Large digital images are usually described in terms of "pixels per inch" instead of referring to the actual number
of pixels that are in the images (it could be in the millions). An image that has 100 pixels across, by 200 pixels
high has a total of 20,000 pixels. If this image had a "document size" of 5"x10", it would have a resolution of
20 pixels per inch. Keep in mind that resolution (pixels/inch) is an algebra problem. It is the result of dividing
the number pixels by the number of inches. If you don't know the number of inches (document size) you cannot
figure the resolution. Likewise, if you don't know the number of pixels, you can't figure the resolution. Finally,
if you only have the resolution and not the document size or the pixels size, you cannot figure the size of the
picture.
This is important to know because you will certainly come into contact with someone in the future who doesn't
understand this, and when they want you to send them a digital image, they may ask simply for the resolution.
An image that has a resolution of 300 pixels per inch could be ANY size image. It could be a 1 inch image at
300 ppi, or a 100 inch image at 300 ppi. Knowing only the resolution is like knowing only the price per square
foot of tile for your house. If you haven't measured your house, you won't know how much tile you'll need.
If you know the resolution (pixels/inch) AND the document size (inches), then you can know the actual size in
pixels--and determine if you have enough of them. Remember that the details (the quality) is determined entirely
by how many pixels you have.

Another technique, and something most profesionals rely on, is to simply know the file size (in KB or MB). Since the number of pixels in your image is proportionate to your file size, then with one number you can know if you'll have enough pixels for the job.

Print Resolution
The general rule of thumb is that you should have around 300ppi for high quality printing. This means that if you want to print an 8x10 inch print, you'll need 2400x3000 pixels. This means that you should have a camera that is 5mp or more.