Hi Richard,
First, the photo must be uploaded to WT, often in a Free Space for photos.
Okay just looked you have a Castles in the Netherland's free space.
This is the easiest way but not the official way. If the image is in that free space, or you can upload it to that space.
Have 2 WT windows open, one the Weekend Chat and the second the free space where the picture is.
Post on the weekend chat, don't finish the post, then just copy paste the photo from the free space to your post.
Or ask for more help if needed.