Are you sure it’s not the image itself? Sometimes certain images display differently in WordPress (any theme) depending on the shape. If it’s too tall/vertical, then the width will be small, and if it’s too wide, the height will be small as well in some cases.
I would try running the Regenerate Thumbnails plugin to see if it adjusts the thumbnail sizing. It usually fixes a WP issue like that, but sometimes it’s the image itself.
If that doesn’t work, can you try a different image? Maybe try one of the same images that already work from the other widgets. If it works in the other widgets and also works in the 3rd, then that means there’s something wrong with the other image you’re trying to use.
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