web image resolution
there are some good sites out there that explain the notion of digital imaging and resolution.
Like this one geared towards digital photographers.
For our purposes though I think that the actual wordpress information is best.
Remember that you can always make an image smaller once it is inserted into your media library, but you do not want to upload enormous image files to your library because they will just eat up your storage space. One nice thing about using flickr.com as a photo repository is that flickr automatically saves all of your uploaded images in a variety of sizes.
For images that I use in my blogs, I generally aim for images around 400 pixels in the longest direction.
Web resolution is typically 72dpi, and occasionally just a little higher. This is because that is the resolution at which your monitor displays/outputs imagery.
A pixel is a unit of digital data.