Tutorial: Digital Art Basics and Image Size Terms
What do "pixel", "screen resolution", "image dimension", "PPI", "DPI", and “high resolution image” mean, and why should you care?
A pixel (picture element) is a single, tiny square of color, the smallest unit displayed on a computer monitor. It is common for digital artwork to be done as a raster-based (aka bitmap) image - that is to say, it is comprised of a grid of individual pixels.
Whether you start with a blank file in Photoshop or import an image from a digital camera, any work you do will be manipulating the pixels that make up the image.
Your computer's monitor display is comprised of hundreds of thousands - or even millions - of individual pixels. Your screen resolution is simply the number of pixels currently displayed on your screen.
For example, if your monitor is set to display at 1280 x 1024, that means your monitor is displaying a grid of 1280 pixels across, and 1024 pixels down. (If you're curious, that's 1,310,720 individual blocks of color!)
Image Dimensions (or Image Size)
Just like your monitor, your digital image will have a size measured in pixels. For example, an icon or profile picture will probably be 100px x 100px or less, while a desktop background image will be exactly the same size as your screen's resolution.
Your monitor cannot display an image larger than its resolution at full size. For example, my screen resolution is set to 1920 x 1080 and I have opened an image which is 5400px x 7200px (Yes, that's BIG, and I'll explain why shortly). Photoshop will automatically show the whole image on the screen, but at a reduced size. Here, you can see that the image is being displayed at only 13.3% of its full size. If I display it at 100%, only part of the image displays, and I have to use the scroll bars to see the rest of the image.
Pixels Per Inch (PPI)
In addition to the size measured in the number of pixels across and down, your image will also have a resolution. This is typically measured in pixels per inch, or PPI.
Images on a computer screen are measured in pixels. The PPI does not matter if you're only going to be displaying an image on a computer! However, if you EVER intend to print your image, you will need to understand how the PPI will affect your image’s printing size.
(Go to the "Image" menu and select "Image size..." to get to this dialog box.)
As you can see, an image which is 72px x 72px, at 72 PPI, will print out as a 1" x 1" picture. An image which is 300px x 300px at 300 PPI will also print as a 1" x 1" picture. Make sense?
Working at high resolution is important, regardless of whether you intend your final image to be displayed on a screen or printed out!
When people say "high resolution", what they really mean is "more pixels". Say you're making an image you want to display as a 300px x 500px picture online. Working at high resolution just means you started with a larger image (say, 900px x 1500px) and then shrunk it down 3x in your graphics program to make your final 300px x 500px image. If you start with a larger image size and then shrink it down for display, you can pack in a lot more details and your final image will look better for it!
I always treat images like I will eventually want to print them out, even if I have no intention of doing so right now. Not only that, but I always treat them like I will want to print them at a very large size (typically 18" x 24" - 5400px x 7200px at 300 PPI). Why? Because it's better to have too many pixels than not enough! You can always adjust an image's dimensions down. However, you will lose image quality if you try to adjust the dimensions up. Here's what happens if I try to upscale the avatar I use on deviantArt:
The effect is much different when I start with a larger image and work my way down:
When setting up an image file to print, you really don't want to go any lower than 300 PPI. Why?
Even when you consider both of those will be shrunk down when printing, you can get an idea of how much crisper that 300 PPI image will be!
One final note - Always keep your original, high resolution file, no matter what! You never know when you’ll need it. If you want to post your image online, save off a separate file before you shrink it down.
The moral of this story: When it comes to digital art, work big!