Fill in objects that pop out from background?
Here is one simple approach:

Step 1. Find the background color.  (This is an entire topic in its own right, so if you're not sure how to do this, ask a separate question about it.)

Step 2. Binarize the image so that all pixels whose color is close to the background color are white, and all pixels whose color is not similar to the background color are black.  This will get you a rough approximation to what you want, but there might be holes in the middle of the black shape (e.g., if the cat was wearing a white collar).

Step 3. Apply a suitable morphological operator, e.g., the closing operator.

This approach will have some limitations, e.g., if the background is not flat but rather has shading or a gradient, but it is simple to implement and try.

If the simple approach I outlined above is not sufficient, the broad problem area you are looking at is image segmentation; you should check out the techniques that have been developed in the literature.  There are many, many methods for image segmentation that you could explore.  For instance, you could look at methods based upon graph cuts.