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I'm using Illustrator on this project. My goal is to cut the dual flag image to the shape of the cat's head, like what I figured a simple clipping mask would do.
The cat's head has a transparent background, and the flags are images that I cropped and grouped together. I know clipping masks don't work unless certain path criteria are met, but I'm just not sure what I'm missing. I can't make the cat or the flags a compound path, nor does rasterizing them help.
What do I do?
Clipping Masks in Illustrator require a single vector path or compound path.
It is not possible to create a clipping mask using a raster image. It simply can't happen. If that cat's head is raster, and it sure looks like it, that is the problem.
If your goal is to merely clip the flag, as it currently appears, to be contained within the shape of the cat's head, then you need to create vector path which outlines the cat's head. Then use that path for the clipping mask. Afterwards merely position the clipped art above the cat's head in the stacking order, aligning them correctly.
If the goal is to have the flag use the tonal variations of the cat's head, then an Opacity Mask (2) would be more appropriate. You can create opacity masks using a raster image. The raster image becomes a "value map" for the underlying artwork - dark areas in the raster image are hidden (masked) more than light areas.
In general, high contrast images work best for Opacity masks. The image posted in the question isn't very high in contrast.
What to do? Trace the head image to vector. Clipping mask must be vector. If needed prepare a dark version of the image ensuring the head outline is a single vector shape with no holes. It won't work if there are white pixels in the head area. Expand the trace result and use it as your clipping mask.
Not asked: If you need the cat's whiskers you can edit the edge of the traced head or draw them with the pen separately and use Pathfinder panel's Unite to combine the drawn whiskers to the rest of the head outline. The whiskers cannot be open curves, they must be drawn as closed paths to be useful here.
If this was my job I would consider drawing the whole head outline with the pen. Professional looking background removals in Photoshop are often done using the same idea. There's also the pen for drawing clipping paths.
It's not especially difficult in Illustrator if one keeps the photo as a reference and if a simple enough edge will do; make sure to lock the photo to prevent accidental moving.
