The free PeepSo Foundation will allow you to add a post with a custom background. To enable adding posts with their own backgrounds activate the Post Backgrounds setting located in WP Admin -> PeepSo -> Configuration – Stream Posts -> Post Backgrounds section.
In the Post Backgrounds settings you will find:
You can also manage backgrounds in WP Admin -> PeepSo -> Manage -> Post Backgrounds.
You can add your own background by clicking the Add New button at the top right of the settings.
After adding a new background, you can:
To add a post with a background on the stream select Post type: Post background from the options available in the postbox.