Your final image should look something like this, where all of your layers are visible. The original snapshot image below is 1200px x 921px, which is a bit large for ordinary web display but good to have to see the details on closer inspection. I selected medium under image size during the WordPress upload image and kept the link to the media file so that it will still be clickable for closer view.
The screenshot below was taken with Jing, free screenshot software for Mac and Windows but you can use any software you like. If you use a Mac, you can take a screenshot of a selection of the screen by clicking cmd+shift+4 and dragging anywhere on the screen. If you use Windows, you can click on the print screen button and then open the screenshot in Photoshop and crop. Just make sure your screenshot is good quality (at least 1000px on largest side)
Also include detailed notes of the process you went through to create the image mentioning any problems you ran into as well as your inspiration for the image.