Should I include funny pictures in my slides?

Human cognition is a strange thing, and I won't pretend I know it well enough to give you a guide, but I will suggest that you simply consider your audience and how you want them to move through the information.

A dense, heavy talk with few breaks will tire most audiences.

A light talk that doesn't require much thought will result in many people letting their mind wander.

A talk which carefully weaves the "story" with a variety of dense to light moments, may actually improve retention.

Adding levity in the form of a joke or humor, if the joke is very relevant to the talk, can actually increase retention, as long as it isn't too much of a distraction. Placing it appropriately is key, though, and I wouldn't do it at the end, because that's what people will remember. The last few sentences should be a quick summary of the talk and should be memorable.

Placing it in the middle, during a transition - for instance between the problem statement and the methodology - could be good, and allow a release of tension if the problem statement was pretty densely packed.

Starting off with it, as an into to the topic and a method to get attention is also a pretty good choice.

In all things, though, consider your audience first. What is the journey you are taking them on, and when would be an appropriate point for a rest break, a transition, a wake-up, etc.

I don't object to have fun, but I would object to have fun when you need to be serious. Because when the audience need you to be serious to your talk, then a funny picture will disrupt the thinking and they will be anxious.

If your talk in short, then I suggest you to put the comic at the end of the talk. If your talk is long, then I think putting it in the middle of the talk is fine, as long as the picture appears when no thinking is required, e.g. the transition between two sections. Not from section 2.1 to 2.2, but from 2 to 3.

(Personally I would put the funny slide after the discussion. Not only saving the best for the last, but also at that time, no one will think anymore, and indeed they are needed to be relaxed.)

A big part of making an effective presentation is understanding your audience. You are going to be the best judge of how a funny slide will be received, since you know your group better than any of us do.

I can tell you that funny slides are not always a bad idea, just like they are not always a good idea - it depends on your audience and the goals of your presentation.

If you do use a funny slide, do it for a reason - but that should be true of all your slides. Every slide should have a clear purpose and be designed to communicate something to your audience (preferably one thing per slide). Funny "thank you" slides are no different. Decide what the purpose of your "thank you" slide is, and whether this comic will communicate that purpose to your particular audience, and then you will have your answer.