One idea is, to add Header and footer images in the Content library through QMC and when you are ready to do the storyboarding may be you can drag the images (through Media objects in the right hand side) and fill the other content as required. Hope this works for you.
If it's a Qlik Sense Desktop free version, just paste the header and footer image into the Qlik installation folder generally it will be like (C:\Users\<Username>\Documents\Qlik\Sense\Content\Default\Images). And then images can be dragged on to the canvas from Media objects.
