Thanks for your suggestions. We have created a web service (Django DB) on a remote system that allows users to upload images using cameras of iPads and smartphones (HTML 5 native interface; works surprisingly well). After an image has been uploaded, we use the XWiki’s REST interface to automatically append them to an XWiki page (in a particular labbook context) and display a scaled version. It is a bug in XWiki: the images are displayed correctly in a number of image viewers (including Photoshop), XWiki apparently does not handle JPEG-images with "Orientation: 6 (Rotated 90° CCW)” correctly. We now use a workaround and rotate the images before sending them to XWiki from our external server with a solution similar to this: ios - Rotating an image with orientation specified in EXIF using Python without PIL including the thumbnail - Stack Overflow