Check your mail please. This problem with file permissions, not with "ARI Image Slider".

Regards,
ARI Soft

The problem is fixed. Thanks for the mega quick help!

To help other if they have the same:

The "Use thumbnails" setting created this effect, it was set to "Yes" creating and using a thumbnail (of the same size as the original) for the slider.

Secondly, because the file-permissions where incorrect after a manual FTP upload the original images where not shown when the thumbnail setting was changed to "No". The image where shown before because the thumbnails where generated automatically (and with the correct permissions).

I used this command to correct my FTP uploaded images for the slider:
chmod 644 sliderimage.jpg