Max width image size that can go into Notices?

[XFFA.IR Bot]

I'm looking to put an image in the Notices. But I want it to cover the whole Notices box. If you look at the following screenshot, you will see a Notices box but it's only covered full in height but not in width, as you will see there is plenty of space on the left and on the right. What image size should I use here to fully cover the width as well?

Also, how to make it responsive? The theme is fully responsive, all other banners are fully responsive but when I put the same code into the...

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