Hi Juraj, in fact the first image comes from
OpenFootage.netI bought for only 5 euro the HDRI (perfect for my project), but 10K isn't enough.
When I render my project in 4K, the mountain HDRI is blurry, like out of focus. But the quality is good (no noise/pixelation/ artifacts on the blue sky for example).
Maybe a 360° Panorama JPG is enough , but in high resolution.