All you need to do is send them a Tmail that says something like 'I was was looking at your lovely pictures of Dietes iridioides, I think they are actually of Dietes grandiflora because........ . If you agree, would you mind proposing that the pictures be moved to Dietes grandiflora? Or, if you don't have the time, may I do it?'
Almost every time, assuming you have good reasons, people will happily agree to move the pictures.
On the other hand (and this is why I'm being so difficult
), asking for pictures to be moved in a public forum such as this inspires a defensive reaction (completely understandable) in at least some people. Simply moving the pictures without saying anything at all to its owner often really ticks them off (also understandable). So, although it may not seem like it, the simplest way to resolve these situations really is to contact the owner(s) of the pictures directly.