Hi @Andreas_Beck - I see that this is your first time posting in the Community. Welcome! 
A quick option you can do is to screenshot your chosen section of an image or PDF in the workspace.
A second option is to capture an image from a PDF in the workspace. To learn how to do so, click here. Note: You will have to download the image to your computer, use an external crop tool, and re-upload the cropped image to the workspace.
If you are on PC:
- Select the image you captured and downloaded from the workspace.
- Go to Picture Tools > Format tab and select Crop.
If you are on Mac:
- Right-click the image you captured and downloaded from the workspace.
- Go to Share and select Add to Photos.
- The Photos app will open. Double-click your image > Edit and select Crop.
If this is a feature you’d like to see in the future, feel free to share this in Product Feedback.