Use of Esto v. Este or Esta
Rosetta Stone uses Esto in the following way:
Esto es una pintura de un hombre. Why isn't this: Esta es una pintura de un hombre? Also in many other examples where the sentence structure is : This is a cat. or This is a dog. etc,. Esto es... is used. Why is this correct?
3 Answers
I think Manity is right. To add to that, it's not incorrect to say ¨éste es un perro¨ in a certain context. For example:
He aquí dos mascotas. Éste es un perro, y ése es un gato. Here are two pets. This one is a dog, and that one is a cat.
Esto es un perro. This is a dog.
When you want to say ¨this one¨ or ¨that one¨, you would use éste or ése, or ésta or ésa, etc. Generally when you want to say simply ¨this¨, you would use esto or eso. I´m sure I'm not stating the entire rule, but this is basically how it works.
I thiiiink... esto is the "generic" word for "this" and it's used as a subject in your first sentence. There's no rule to have the subject agree with the direct object (una pintura...).
On the other hand este and esta are demonstrative adjectives (or pronouns) and they need to agree with the word they describe. So: Esto es un perro. Este perro es blanco.
(I might be wrong about this, I'll gladly accept corrections if this isn't right.)
Esto is a noun. Este/esta is an adjective.
Esto es mi silla / This is my chair.
Esta silla es mía / This chair is mine.
Esto? Este estadio está al este de esta estación en estambul. / This? This stadium is to the east of this station in Istanbul. 
