What is the legal status of using copyrighted images in academic/conference talks?
Legal status can vary by country, content and sometimes even - the intended audience.

In many places you are allow to use otherwise copyrighted materials within fair use, e.g a figure from a paper.

For things, which are not directly relevant to talk (e.g. "let's put a nice stock image of a man with a plant") the story may be different.

In any case:


from the practical perspective, you are unlikely to go to jail (or even: getting fined) for showing copyrighted images in an academic talk,
from the legal perspective, unless you are a lawyer (or your country has a very clear law on copyright) you are unlikely to know wether is is legal or not. 


Highly relevant:


Can I use images showing up in Google search for my presentation slides without violating any copyright?