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http://www.catseyecattery.co.uk/testimonials /
... and I'm happy to report that she is a picture of health!

Is a picture of health correct? Does it mean the same thing as "the picture of health" in "She is the picture of health. [=she looks very healthy] (http://www.learnersdictionary.com/search/picture )"?
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The meanings are the same; they are equal variations.
Comments  
Students: We have free audio pronunciation exercises.
It's "the". "The picture of X" is a fixed expression. Kofi Annan is the picture of a diplomat, not a picture of one.
COCA gives me 8 'a picture' and 39 'the picture', so the latter seems to be, though not statistically, more common.
I don't think that if enough journalists make a mistake it becomes standard English.
Students: Are you brave enough to let our tutors analyse your pronunciation?
I don't think that if enough journalists make a mistake it becomes standard English.-- Oh, it's well on its way, though. I dread to think how many changes in the language have been brought on by the media.