You can only add an illustration to a chapter which you have uploaded or purchased. To add an illustration to a chapter, you need to be logged in to www.thecreativetree.com. Then, go to your My Account page (you can get there by clicking the "My Account" text at the top right of any page) and follow the instructions in Stages 1 - 4 of the Add Chapter FAQ (https://www.thecreativetree.com/about/add-chapter/).
When the reading window for the chapter you want to illustrate pops up, Click on the "Add Illustration" tab at the top of the window. See the screenshot below:
The "Send Illustration Request" window now pops up. To protect authors from having their chapters flooded with illustrations which they may not consider appropriate, you have to send the author a request to add your illustration to their chapter (even if the author is you!). The system will show all the photographs and graphics which you have uploaded. Select one of them by clicking on the radio button to the right of it, as below:
Once you have pressed the "Send" button (at the bottom right of the window), the author will receive your Illustration Request and can choose whether to Accept or Reject it.