.... what is an icon?  what is still an icon?  what is art, and what is just a little religious painting? 

Artists are lucky.  All they have to do is paint a representative “old chap” with a philosopher’s beard and a sword, the rest is painted in accordance with the fashion of the time.  And so, the modern Church has yet another portrait of the Paul the Apostle.  
But what if the sword is missing from the picture? Then, all that remains is an old chap with a philosopher’s beard.  Without his sword, the holy apostle becomes invalid! 
The icon neither has, nor knows, nor needs any special attributes; all accoutrements for the purpose of being recognized are superfluous.  From the start, its personal similarity is retained through the love for the Saint represented. 
It makes no different where the icon comes from or how old it is; an icon of the holy Apostle Pual will always be just that, and will bear his name.
The inscription which marks the icon’s completion is, too, proof, evidence that the icon is the property of the saint portrayed. The inscribed icon must be consecrated. By the power of the consecration mystery, the icon is taken into the service of the Church!  Christians are convinced that the inscribed and consecrated icon actively participates in the church service through the presence of the represented saint.

Mother of God icon

always still a Mother of God icon

Not an icon, but a religious picture

portrait of a woman with child

Also a Mother of God icon

Has similarity with a Mother of God icon

a religious picture

Another Mother of God icon

Hardly a mother of God icon

a devotional picture