Header Image blurry/distorted

Hi,
during the usage of the plugin I always run into the this problem:
If I upload an image for the header (all other images are fine) it always seems to be blurry. After some troubleshooting I found the problem – the height is always -1 px to the image height (e.g. 600x140px image gehts width: 600px; height:139px;).
Because of this the image gets blurry. I also tested this with other image sizes and the same problem occurs (turning off all other plugins-> nothing changed).
Normally I would write my own changes to the css, but it seems thatthe plugin does not load the style.css of the theme.
Some help would be appreciated.
Regards

https://ww.wp.xz.cn/plugins/wysija-newsletters/