I need some sort of dataset which has related items in it. For example, a flower has related subtypes: roses, violets, etc. Each of those subtypes has their own subtype. This could be a graph of related items which is used in semantic search engines, etc.
Is there anywhere that has such dataset (preferably with images)?
Wordnet would be a good start. You can get if from here for free.
Conceptnet is another great ontology. It has a lower quality, but a much larger number of concepts. Here's the Conceptnet page for flower
The third source I'd recommend checking out is wikipedia cross-article links.