When we talk about the dimensions of an image we mean the image's width and height. Digital images are measured in pixels, for example, 900 pixels x 1200 pixels. Pixels are the tiny colored points of light that make up computer monitors and other displays; they're the smallest visual units of a digital image.
It's important to realize that an image's pixel dimensions are not the same as its absolute physical size. The apparent size of an image is dependent on the specifications and settings of monitors, displays, and output devices.
The second factor to pay attention to is resolution, which is expressed as pixels per inch, or PPI. Resolution is a measure of the density of the pixel grid used to display an image. The higher the resolution of an image, the more pixels are used to represent it--and thus the more details that can be captured and represented. Different media require different resolutions. A printed image, for example, requires a higher resolution than an image that will display on a monitor.
The last factor to consider is file size. File size is measured in such units as bytes, kilobytes, and megabytes, and it indicates the amount of data stored in a file. This data is what is used to recreate your image onscreen. With digital image files, more data generally translates to greater detail and, potentially, to a crisper image. But remember that larger files will take longer to upload and download, so, when you're preparing an image, it's important to strike a balance between file size and image quality.
Image compression utilizes complicated formulas to systematically eliminate data, allowing you to reduce file size and yet maintain image integrity and quality. Getting rid of too much image data can result in images that look "pixelated" or blocky, so be careful not to over-compress your images. JPEG is one of the most common file types used for compressing images that will be posted online.