1. To make changes to the background of your post, click on Edit Image in the bottom bar of the post.
2. To remove the background, click on Delete Background in the right column.
3. For a different background, go to Elements in the extreme-left panel.
4. You can use a photo, texture or a pattern for the background. Choose the desired element.
5. Click on Make Background in the right column to make that photo / texture / pattern the background.
6. Save your changes once you're done.
And you're done!