Recognize oval shaped illustrations in scanned book pages

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i like your ideas :-)

In addition to this, I would suggest blurring your images first with a Gaussing blur and then try to fit an ellipse. This will ensure that you don't fit ellipses to shapes inside the image.

I tried this and it looks promising. The example has the drawback that it only looks for the best match, even if that match is rather poor (for example, in a page without portraits). I settled for CV_TM_SQDIFF_NORMED and skipped the normalization. Then I tried to pick out the best candidates. However, my medium-grey mask missed some of the portraits (that were dark grey) and instead suggested hits in the text of the page. I think the page image needs preprocessing for this approach to be successful. Perhaps erosion can make the text go away.

Take a look at "How to handle template matching with multiple occurences":

http://opencv-code.com/quick-tips/how-to-handle-template-matching-with-multiple-occurences/

And you can also have two templates (one for the "normal" cases , and another for some "peculiar" cases with bad illumination , blurred images or dark grey).

As preprocessing step you can blurr your images first with a Gaussing blur in order to deal with the text of the page (as Steven suggested).