iTunes won't update podcast artwork

The feed has indeed updated to the larger image, but it has an alpha channel and I would expect Apple Podcasts to reject that. You should flatten it (saving it as a jpg would do that)

The image URL in your 'itunes:image' tag is https://bibelnerden.no/blog/wp-content/uploads/2021/09/logo2.jpg

This image is 949 x 951 px: it must be minimum 1400 x 1400 px and it must be square. This is the black and white image: Apple Podcasts 'browse' on a Mac is showing a green and white image. However when subscibing on a Mac I see the black and white image: I'm a bit surprised as the image isn't an acceptable size (perhaps subscribing is less fussy). Perhaps 'browse' will catch up (it can take some days) whereas subscribing reads the feed directly, but you should probably remake the image to the right size.