Up to now, i always have used the "/Bild" ("/Picture") and in the picture posting dialog which opened then, i have uploaded the file. With the result of the error message "File could not be saved".

NOW i have tried it the first time, with drag & drop directly into the message form.

If you can see an image under this line it works this way:

Thanks! 

(Anyhow, the error in the other way should be fixed. There is no info, that images can be used via drag & drop)