Typically images are licensed not sold - that means you are not selling rights to the image but permission to use the image in a certain way, situation were a photographer sells the copyrights do exist but they are relatively rare and expensive.

The image license is usually priced by multiple factors - how much time the image will be used, where it will appear, the size, quality and importance of the image, number of places it will be used, etc.

For example, low res copy that you can only use on Facebook once - cheap; high res copy you can use forever in magazine ads - very expensive.

You will need to write a contract that say, for example, you are selling a license to use the image on up to X internet sites with a maximum resolution of X for a period of X years.

A fair price should be more than it costs you to make the image (including direct expenses, your personal expenses, your profit you need to make, and all the expenses of a business divided by all your sales - google "cost of doing business") and less than the extra sales she expects to make because of the image - tools like FotoQuote can give you a starting point.