@Nagnag – yes. Open the images in PhotoShop and save them before running the data merge.
Depending on the PhotoShop version you are using there are several options to batch save a whole directory of images.
Keep in mind that your camera only has one orientation for capture and you must have physically rotated the camera to change the oreintation of the image.
Thank you Uwe. That worked perfectly!
Do you know why it works this way? In all other applications (Bridge, Finder) the photos do show as portraits.
@Nagnag – I do.
The simple answer is: Every time you are shooting an image your camera is writing metadata. Among deverse information like camera specifics, time the photograph is taken etc.pp. there is also a flag that points at the orientation you have hold the camera. That is working in most of the cases.
PhotoShop can read that information properly and will open the image according to that flag in portrait or landscape orientation.
InDesign cannot read that information and always will place an image coming directly from that camera in one single default orientation.
Now the solution: if you open and save an image with that special flag, PhotoShop will write the right orientation to the image so InDesign can place it like the orientation is showing in PhotoShop.