Fucking around with image trace because someone asked how to do this kind of stuff on Reddit.

Process:

1. import image
2. object->image trace->make
3. window->image trace, set it to black and white with a threshold of about, oh, 30 or so. Maybe open up the ‘advanced’ triangle and check ‘ignore white’; I’ll talk about why you might want to do this later.
4. in the layers palette, drag the layer this image is on to the ‘create new layer’ button at the bottom of the palette.
5. you are now editing a new copy of the image, in this new layer. Set the trace threshold a little higher.
6. repeat steps 4/5 until you feel like you have Enough layers to work with. You might want to set these layers to about 20-50% at some point so you can see what’s going on.

Now, you have at least two options here. First I’ll talk about how I did the B&W image with pattern fills.

1. I’d checked ‘ignore white’ in step 3 above. This gave me a set of paths that were just the black parts, as opposed to solid black and white.
2. make a new layer at the top of the stack, call it ‘construction’. Probably lock all the other layers so you don’t interact with them by accident.
3. somewhere above the image, draw a horizontal unfilled, stroked line that’s around half again as long as the diagonal of the image.
4. effect->distort and transform->zig zag. If you want wavy lines like this then choose ‘smooth’ in the ‘points’ section.
5. alt-drag the line to well below the bottom of your image.
6. select both lines, object->blend->make. Then object->blend->blend options and fiddle with the settings until you like the spacing between your lines.
7. select that whole blend and drag it into the swatch palette to make a pattern fill. You could also do object->pattern->make but that will immediately throw you into the pattern editor mode, and we don’t need to do that here.
8. Hide the ‘construction’ layer. Show everything else. Select all the image traces and do object->image trace->expand. Sadly you can’t expand multiple image traces at once; you have to select them one by one. I feel the most efficient flow for this is to unlock one layer, select all, expand the trace, lock the layer, then go on to the next one, but whatever works for you. You might want to record an action or add a keyboard shortcut to the image trace expand as it’s pretty deeply buried in the menus.
9. Lock all your layers; unlock one and do edit->select all. Then pick the wavy lines pattern swatch you made.
10. Lock off that layer, unlock another one. Select all and pick the wavy lines pattern swatch.
11. Choose the rotation tool and start to rotate the image. Before you let go, press the ~ key. This is a switch that says “only transform the pattern fill”; you’ll see your outlines replaced by bounding boxes of all the paths. Maybe hold down shift to constrain it to 45º angles.
12. Repeat steps 10/11 until you’ve dealt with all your layers.
13. Enjoy your cool artsily separated photo. You could use whatever fill pattern you like for this.

(As a side note, the ~ trick to move a pattern fill around in a shape works with the scale, reflect, and arrow tools, as well. It does not work with the free transform tool.)

And here is another way to do it: opacity masks.

1. do those six steps at the top of the post, but turn off  ‘ignore white’ in the image trace options.
2. lock everything except one image trace layer. Select all; cut. Yes, cut.
3. In the layers palette, click on the circle to the right of the now-empty layer’s name.
4. In the transparency palette, press the ‘make mask’ button. Check ‘clip’ and ‘invert mask’.
5. Click on the big black square that appeared in the transparency palette, and paste. Now you have an opacity mask. You can see its outline if you do view->show edges, and you can see it in the transparency thumbnail’s palette, but nothing shows up on the screen. That’s because the opacity mask is a greyscale image that affects the transparency of what it’s attached to, and right now it’s attached to an empty layer.
6. Click on the empty square in the transparency palette to go back to editing the layer, and draw something in it. Maybe a big black rectangle. Maybe a colored one. Maybe a pattern fill. Maybe draw two circles and blend them, that’s what I did here. And then duplicated and slightly offset them to create a cool morié pattern in the image.
7. Repeat steps 2-6. If you want to move some of the stuff you drew in step 6 without moving the opacity mask, then click the chain link between the two thumbnails in the transparency palette. Otherwise you’ll move the opacity mask as well.
8. Enjoy your cool artsily separated photo. You could also try unchecking ‘invert mask’ on one layer and using it to overlay art in the lighter areas of the image, rather than the darker.

There are other ways to do this – you could expand the image trace (with ‘ignore white’ on), turn it into a compound path, and use it as a layer clipping mask for the layer full of whatever imagery you want to draw; you could probably do something involving destructive operations with the Pathfinder palette, too. Which is a major sin in my book as I like to do non-destructive edits whenever possible.

If I was to rank these methods from most to least editable down the line, it’d be opacity mask > pattern fills > layer clipping mask > pathfinder stuff. For instance, I wanted to add a little extra shadow under the chin to help distinguish it from the face. With the opacity mask method I could just go into a layer mask and draw some black shapes over the image trace. Adding more shapes the pattern fill way can be a little finicky with keeping the fill patterns in alignment; adding more shapes to a complex layer mask is even more fiddly, and destructive pathfinder operations have to be done completely from scratch.

You can also do a similar trick with Astute Graphics’ WidthScribe palette, if you feel like spending some money for a plugin that only has a bunch of YouTube tutorials and no manual.