Announcements is a new feature currently rolling out in Teams, Announcements help you to make the conversation standout in the channel. There is a main heading field as well as a subheading along with the option of a background colour/image able to be applied.

To start, click the Format icon under the new conversation box:

Traditionally, you would have then seen:

But now, you would then see the option for Announcement under a drop down menu:

You can have a coloured background:

Or pick a picture:

You can then pick the part of the picture you’d like as the background if it’s bigger than the minimum size

Once posted, there is a new icon to show that it’s an Announcement

At the moment there doesn’t seem anyway to restrict this, but that may come in a future release.