Yes i know its a little bit difficult, and i guess it´s some kind of a bug.
But on this following Way it works
1. Choose the Icon with the FloppyDisc in front of the Picture "Host an Image"
2. A Screen opens where you can choose your Pic Location with the Button
3. Choose your Pic
4. If you like to, you can Resize your Pic. If you don´t want to resize, just choose "no" from the Dropdown
5. Click "Host it"
6. After Upload, three URL are shown in the Window
7. Choose the last one and click "COPY"
8. Now click on the Icon right beside the "Host an Image" Icon.
9. Paste the URL to the Field and click ok.
10. Now you´re Screenshot will show up
Hope this helps
GRONI