Center an image in a text widget

I have managed to put a photo in a text widget, and make it into a link, but I can’t figure out how to center the image within the text box. I’m sure this a simple CSS thing, but I don’t know CSS. Would appreciate any pointers.

It’s the image of the book “Photographing Oregon” at
https://www.gregvaughn.com/blog that I’m referring to.

Thanks,
– Greg