DPI means a lot to printing, but really has nothing to do with the actual image in a file. If, for instance, you post some images to FiberglassRV and each one is 300x400 pixels, they'll all appear the same size on the screen. If you tell your photo or graphics program that one is 600 DPi, it will print it 1/2" wide; tell the same program that another of these images is 100 DPI, and it will print it 3" wide... but they're really the same size of image data, and will display the same on the screen in most programs (such as browsers). The DPI setting can even be stored in the image file, but the program which displays it (a browser or a printing program) can ignore that, and display or print whatever physical size you want.
An example... (left one is 600 DPI, right is 96 DPI, both are 300x400)
For printing, you should consider DPI, as Mary has explained. For web posting, pixels are all that really matters; the size which you can measure with a ruler on your monitor will depend the monitor's pixel pitch. Both of my examples appear 3" wide on my monitor because it has a common pixel pitch of about 100 dots (or pixels) per inch.
Since many people use photo-editing programs which are oriented towards printing, they tend to express the size of images in inches and DPI, but the data is really just pixels. I think this just causes massive confusion.
That's my egg in the photos - sorry it doesn't fit Doug's theme...