The correct option is B I will paint some portraits during the summer vacation.
'Any' is usually used in negative sentences and in an interrogative form mostly where the answer is expected to be in negative. Since the sentence is affirmative, a change needs to be made. Option A is incorrect. Option B is correct because the quantifier 'some' is used in positive sentences denoting a small number of portraits. Option C is incorrect because 'every' is used for singular nouns to emphasize every member of a group, the noun being plural makes the option incorrect. Option D is incorrect because 'a little' is used for non-countable nouns.