This is a very easy-to-achieve effect that I used in a small project recently. If your photo is a little bit comedic, or that's the theme you're going for, then adding a cartoon/comic-style effect might be a good idea.
So I'll be showing you a very quick way to take your photo from this:
and turn it into this:
1. Intro
Firstly, open up a photo in Photoshop, crop it and make any other simple adjustments you may want.
2. Gradient Map
Add a Gradient Map adjustment layer by going to Layer > New Adjustment Layer > Gradient Map, using black to white as the color and click OK.
Now, in your layer's palette, select the Gradient Map layer and lower the opacity to 80% (or so).
3. Apply Image
Create a new layer (Layer > New > Layer) and then apply the image by going to Image > Apply Image, click OK with the default settings.
Now you'll have whatever was visible underneath on a layer of it's own. Make a duplicate of this layer and name them (01 and 02 for example).
4. Film Grain
For your first duplicate (01) go to Filter > Artistic > Film Grain and select some random settings (doesn't really matter). Next, change the layer mode of this layer to something such as Color Dodge and lower the fill to around 30% (or whatever looks suitable).
5. Poster Edges
For your second duplicate (02) go to Filter > Artistic > Poster Edges, use similar settings to these:
Edge thickness: 0
Edge intensity: 0
Posterization: 2-3
Lower the opacity of your 02 layer to around 50%.
This is the main step that makes your photo look more like a comic, so you may want to try different settings for the filter and for the layer.
6. Using it as an Action
If you're going to be applying any effect to a whole bunch of photos, then it's a good idea to look into Photoshop actions. All you have to do is start recording, apply the effect, stop recording, then you can apply it to any image by just pressing a shortcut key.
To open up the Actions window, go to Window > Actions.
7. Finished!
Now we're pretty much done. Now you should add some stuff that you think goes along with the theme. How about some text, a speech bubble or a thought bubble?
More Tutorials: 
