(a) A librarian works in / at the library. / A librarian is in charge of a library.
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All three are correct, but have slightly different meanings.  Also, the switch between indefinite and definite articles in your first two examples sounds odd.

(b) A grasshopper looks / looks like a praying mantis.”

I don't understand the distinction you're trying to draw here.  A grasshopper looks like a praying mantis is correct.

(c) A boy is climbing / climbing up the ladder.”

Again, I don't understand the distinction you're trying to draw here.  A boy is climbing up the ladder is correct.

(d) The water lily plants / water lily float on the pond.
     The water lily plants are floating in the pond.”

The first example is correct as it is, although people usually call them "water lilies" instead of "water lily plants."   The second example is incorrect because the conjugation of the verb "float" does not match the noun.  It has to be either the water lilies float... or the water lily floats... depending on the number of water lilies you're referring to.  The third example is correct.  In this context, either the preposition "on" or "in" can be used depending on your meaning.  Using "on" emphasizes that the lilies are on top of the water (think of a vertical plane).  Using "in" emphasizes that the lilies are in the middle of the pond (think of a horizontal plane).

e) He is picking up the book on the floor.
     He is picking the book up on / from the floor.”

I would avoid using the preposition "on" in this context.  It is technically correct if you want to emphasize "the book on the floor" as the entire subject.  However most people would assume that "on" complements the verb "to pick up". In that case, only "from" would be the correct preposition.  Either He is picking up the book from the floor or He is picking the book up from the floor sounds best.

(f) This is a photograph of my family.”

This sentence is fine.

(g) The boy pointed to /at the toy he wanted.”

Either preposition is correct, but using "to" emphasizes the act of pointing itself a bit more.

(h) She is putting the dirty clothes in / into the washing machine.”

Technically "into" is more correct since the laundry will be wholly inside the washing machine, but native speakers will often substitute "in" for "into" in this context.