The picture is of Betsy Bobbins, not Dorothy Gale 

The picture from the book series with caption: 
"Dorothy, as depicted by John R. Neill, is usually cited by fans of the book series as the way they see the character." 
- is in fact a depiction of Betsy Bobbins. 

Curiously, although the "Summary" on the picture's page clearly states "Betsy Bobbins as depicted by John R. Neill", the picture itself is entitled 'DorothyGaleNeill.jpg', and the uploader (Scottandrewhutchins) declares that it is of "Dorothy as depicted by John R. Neill".  

The young girl is clearly a straw-blonde or redhead like Betsy, and quite unlike other descriptions of Dorothy, so I'm really not sure how this could have happened. 

SDoradus