You can easily export a document with transparent background.
To export a document with transparent background:
Click the toolbar button (Command-Shift-I) to open the Inspector.
Switch to the tab and remove all layers of the page by clicking the close button left to the layer title:
Make sure the and options on the panel are switched off:
At this point the document background should not be visible any more:
Click the toolbar button (Command-E) and choose . When Posterino asks you for a filename and format for the image to export, make sure to choose PNG or TIFF as format:
JPG images can't have transparent backgrounds.