This looks like a regression. I tested on nightly 2.9.x and do not see this behavior. On 10.x nightly, trunk, and Longsight QA05 I see this behavior.
1) Add a large image to the Resources tool. I'm using an 874 X 1024 image.
2) Add the Announcements tool to your site.
3) Go to Home. Edit Site information display. use the image tool to add the image from Resources. Save.
The image bleeds over into the synoptic display for Announcements.