It's supposed to take after whatever you configure the Content Background to be, which is also the same background setting used for your visitor messages and About Me section. What's the image you're trying to use?
I'm not sure. I did several tests and it worked as expected almost every single time:
The only time I managed to make the background image invisible for that part is when I'm not logged in. Otherwise, it should appear, both on your own profile and others', even as a regular user.
That said, in the forum software's history, profile page customisation is an ages-old feature that predates the activity stream (which, comparatively, is rather new). It's possible that there are cases in which profile page customisation fails to properly account for the activity stream. I'm just not seeing it in this particular case.
[MENTION=2]Ignis[/MENTION]; i have the same problem also my self with those white boxes at the Activity. I will post also and my setting to tell me where or what i am doing wrong.
[MENTION=2]Ignis[/MENTION]; late update: On my new laptop I installed a new chrome browser, and this issue seems to be not appearing anymore. So it's probably with the old chrome version I used (: http://prntscr.com/knhcy5