When using an image as backgroundimage, there is currently no way to "play" with the contrast/transparency level. As a workaround I design my backgrounds myself in Photoshop, but it would be great if PowerApps provides this capabilities itself.
In the tools box for the background image there are no tools available to set the transparency level of the background image. A box or better, a box and a slider (too many people don't know that the number in the box needs to be between 0 and 1) so you can set the transparency level. Or even a fade option would be nice