Hello,
This is visibly a problem related to windowing. Your image has values that are beyond the [0,255] range of a grayscale 8bpp JPEG image.
The forthcoming release of the DICOMweb plugin for Orthanc will allow to specify the “windowing” option for WADO-RS Retrieve Rendered Transaction:
http://dicom.nema.org/MEDICAL/dicom/2019a/output/chtml/part18/sect_6.5.8.html
Thanks to this “windowing” option, you’ll be able to adjust the contrast of the image.
Sébastien-