After you have clicked on Edit Image at the bottom of your post, you will land right inside the designer.
Follow these steps to add a Pattern to your layout:
1. Click on Elements, and then select Pattern.
2. Choose the pattern you like and click on it to add to your layout.
3. You can easily make this pattern your background by selection Make Background.
4. You can upload your own Pattern as well by clicking on Add Image
5. Select Change (right panel), and then choose My Uploads. You can upload any image up to 2Mb in size, .jpeg/.jpg/.bmp/.png.
7. Click Done and then Save after you have made your edits.