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 Texture to your layout:
1. Click on Elements, and then select Textures.
2. Choose one and click on it to add to your layout.
3. You can make this texture your background by clicking on Make Background (right panel)
4. You can upload your own texture image by clicking on Add Image.
4. Then select Change, followed by My Uploads. You can upload any image up to 2MB in size (.jpeg/.jpg/.bmp/.png)
5. Click Done. You can repeat step 3, if you would like to use your texture image as a background.
6. Select Save after you are done with your edits