I saw that this tutorial had been briefly linked to in a comment a while ago, but for those who haven't seen it, this is a tutorial for PS7 about how to make a photograph look like a comic. Here is our end result:
Ok, so I started with this cap from Buffy - Doppelgangland:
It is important that you choose your image carefully. This effect won't work well on poor quality. You want a strong image with strong lines. Also, it's important not to shrink it down, because you need as much size as possible so the effect works. The first thing I did was play with the levels. You want a good contrast between light or dark. I decided on this:
But have a play and see what works. I then put two filters over the picture. The first was the Median filter at +1, just to give it slightly cleaner lines. The second was the 'poster edges' filter (Settings: Edge thickness +2 Edge intensity +1 Posterisation +2). This came out looking like this (from now on I'm only going to use a detail of the overall):
This effect is flawed, however, so you will need to see what important lines it missed. It often ignores the bridge of the nose or the line of the jaw, so often you have to draw these in. A good tip is to create a new layer (so that if you make a mistake it can be played with) and then click along the line you want to create, holding down the shift key. This will create a straight line between the two points you click, and help you draw neatly without having to draw freehand.
As you can see here, I've added to her jaw. It doesn't have to be perfect, as we'll be fooling with the image more, but it does need to be reasonably accurate.
If you've created a new layer, merge the whole thing and then the extreme part! Go to Image > Adjustments > Threshold. This will bring the whole picture down to blacks and whites. Play with the toggle to adjust the black/white ratio until it looks right. You don't want too much white, so leave some good blocks of shadow. This is what I chose (threshold level 71):
Then you need to colour in the blacks and whites a little, because the posterize filter tends to leave specks of black. This is what I did:
As you can see, most of the work I did was around Evil!Willow's mouth and eyes, just tidying it up a bit.
Ok, now it's time to crop to the frame that you want. We will be working from this:
Then you need to click "Layer > New > New Layer from background". Then create a new layer and fill it in with white, dragging it to the bottom of the layers as your background. Then, select the magic wand tool (make sure it is NOT set to 'contiguous'), click on the main layer (not the new blank one you created)selecting all the white sections, and delete it all. This layer should now only have the black lines, meaning you can colour it in without having to be too careful.
Now to colour in. To make the funky Lichtenstein-style dots (quite subtle but fun on Willow's face), select the round paintbrush (7px) and click on the 'Brushes' tab in the top right. Click 'Brush tip shape' and drag the 'spacing' bar until it looks right. Here it is at 176%.
Make sure you have your rulers on (ctrl + R). Create a new layer and select the colour you want. Holding the shift key down, click the paintbrush once, noting where you are on the rulers, and then move down as far as you wish and, holding down the shift key all the while. When you are down - and level, according to the ruler - click again. You should get a row of dots. Then simply duplicate the layer and arrange as you wish. Erase the areas you don't want to be dotty - remember eyes and teeth if necessary.
Then colour in the rest of the picture on your white background.
Once you've done that, you can finally shrink it to the 100x100 pixel size LJ icon:
Draw a black, one-line border around the picture to give it that cartoon box look, and then add speech bubbles or boxes as you wish. Speech bubble can be found in the shapes tool.
Here I've added a black, 2px "stroke" to the layer (Layer > Layer style > Stroke), again in-keeping with the comic style.
Then add your text. I use Gnatfont from dafont.com
And there you go; your final result:
Quick tip: if you want to flip, shrink or warp an image, select its layer and press ctrl + T. This will bring up a bar along the top that has various options. From there you can skew, rotate to a specific degree or shrink/stretch to a percentage. However, if you right-click on the image at this point, it will bring up a menu including "flip horizontal" or "flip vertical" - this is a much quicker way of flipping and image, and one I used here on the speech bubble, which is usually the other way around.
This effect can be achieved on any good image, and it's really cool. I like to add text that makes it look like the episode was made into a comic, but you can do whatever you like with it. Other examples:
Hope this helps!