Not an ideal situation, but if it's too complicated to get the pictures posted, maybe use a service like http://postimage.org/ or https://imgsafe.org/ and post the links to the pictures.
I just noticed myself that if a picture contains multiple dots in a filename (eg time) nabble doesn't know how to name it and only uses the root name with the extensions. This way pictures get all renamed to the same internal filename and only the last uploaded picture gets used. So it may be needed to rename the pictures before uploading.
And I've just realised that in my response to Harry I failed to say you must login first.
If you're not logged in you don't get the toolbar above the message entry box that includes the "Insert Image" button.
Mind you, now I think about it, that only matters on other Nabble forums where unregistered users are allowed to post. Many visitors forget to login in don't see the toolbar and also complain about always having to complete a Captcha.