Hi there!
OK, I found the solution! Instead of 'Post Quick Reply', choose 'Go Advanced'. Scroll down, and in the 'Attach Files', click on 'Manage Attachments'. Browse a file from your PC (I will upload Luke & Leia), click on the 'Upload' button! Be careful, because there is a file limit, for e.g. max 256 KB for jpg files!!!
Here comes the trick! If upload was successful, you will see in the 'Manage Attachments' window a new box named 'Current Attachments'. Click on your file!!! It will open in a new tab in your browser. Copy its link!!! This is an 'inside' XDA link.
Now click on the 'Insert Image' icon here, and when it asks for your link, copy your XDA link, and that was that:
You can reduce the photo in Photoshop сс 2017, read about it here:
FixThePhoto