That’s correct. First image unless there’s just a onebox in which case the main target thumbnail image is selected. This is up to onebox code, but is usually determined by the source site tag in:
<meta property="og:image" content="----here----">
There’s no way to select which image manually in TLP, apart from support for a default image.
Richer behaviour is an enhancement and has been requested by the community. First change would be to allow one to select an image. I have a couple of sponsors already for that effort. More welcome.