Hi Pierre,
The dark dashes in your attached image are definitely JPEG artifacts, which is a side effect of the forums compressing a very large image down to under the attachment size limit (200kB). You can see they form a lower-resolution grid than the image's native resolution.
Don't rely on the forum software to compress your image. I'm not sure what options NX studio has in this regard, but you could use free software like
GIMP or
Paint.Net to compress the image as a JPEG under the 200kB limit yourself, which would get rid of most of the artifacts at the same resolution.
I prefer Paint.Net for this purpose, since you can adjust the compression parameters and get instant feedback on the final quality and file size.