I do not really like the idea of Twitter taking a leaf from Facebook’s profile banner. But the change has happened and I have moved on with the change.
There is no one-stop Twitter banner creating tool online at the moment (none that I know of anyway). But I found this wonderful template that highlights the visible and invisible areas on the desktop and mobile versions of Twitter.
I recommend flattening the PSD and using it as a semi-transparent layer to guide the resizing of your favourite image and the positioning of text or other artefacts.
I recommend using your own photos or searching for high resolution CC-licenced images with ImageCodr.
The actual amount of usable and visible space is reduced if you design for the mobile platform. But given how often tweets are viewed on mobile devices over desktops, it would be wise to design for mobile views.