1. Wisconsin
Is this illegal? Yes, technically.
12.13 Election fraud.
(1) Electors. Whoever intentionally does any of the following violates this chapter:
...
(f) Shows his or her marked ballot to any person or places a mark upon the ballot so it is identifiable as his or her ballot.
...
via Wisc. Stat. § 12.13(1)(f)
2. Maryland
Is this illegal? Unclear.
Md. Code Elec. Law § 9-217 states that "[a] person may not ... distribute, possess, print, or reproduce a ballot other than as authorized in this article." Doe v. Walker, 746 F. Supp. 2d 667 (D. Md. 2010), limited the application of this statute to the extent that it conflicts with the Uniformed and Overseas Citizens Absentee Voting Act, but no other interpretation of the statute can be found. It is unclear whether a Maryland court would consider the photography of a ballot to be a reproduction or possession of a ballot in violation of this section.
3. Alaska
Is this illegal? Yes.
Subject to AS 15.15.240 a voter may not exhibit the voter's ballot to an election official or any other person so as to enable any person to ascertain how the voter marked the ballot.
via Alaska Stat. § 15.15.280
4. Texas
Is this illegal? It depends.
Texas Elec. Code § 61.014 prohibits the use of recording devices within 100 feet of a voting station. However, mere disclosure of one's own vote is not prohibited; see Texas Elec. Code § 61.006.
5. Massachusetts
Is this illegal? Yes.
Whoever, at a primary, caucus or election, places any distinguishing mark upon his ballot, or makes a false statement as to his ability to mark his ballot, or allows the marking of his ballot to be seen by any person for any purpose not authorized by law, or gives a false answer to or makes a false oath before a presiding officer, shall be punished by imprisonment for not more than six months or by a fine of not more than one hundred dollars.
via Mass. Gen. Laws ch. 56, § 25
6. New York
Is this illegal? Unclear.
§ 17-130. Misdemeanor in relation to elections. Any person who:
...
10. Shows his ballot after it is prepared for voting, to any person so as to reveal the contents, or solicits a voter to show the same
...
via N.Y. Elec. Law § 17-130
But...
@NYCMayorsOffice is it illegal in NYState to tweet/instagram a photo of your ballot/vote? I can't find a definitive answer.
@NYCMayorsOffice is it illegal in NYState to tweet/instagram a photo of your ballot/vote? I can't find a definitive answer.-- William Bright
@brightfactor @NYCMayorsOffice MT @PolancoPolitics no. But don't delay lines by taking pictures- sites are busy, glad you are engaged .
@brightfactor @NYCMayorsOffice MT @PolancoPolitics no. But don't delay lines by taking pictures- sites are busy, glad you are engaged .-- NYCBoardOfElections
7. Kentucky
Is this illegal? Unclear.
Ky. Rev. Stat. § 117.236 prohibits recording the identity of voters within the voting room, but it is not clear that this would prohibit a voter recording his own vote.
8. California
Is this illegal? Yes.
After his or her ballot is marked, a voter shall not show it to any person in such a way as to reveal its contents.
via Cal. Elec. Code § 14291
9. Minnesota
Is this illegal? Unclear.
§ 204C.17 states that "a voter shall not reveal to anyone in the polling place the name of any candidate for whom the voter intends to vote or has voted" (emphasis added). It is unclear whether Minnesota courts would apply this section to photography which subsequently reveals to the public how the voter has voted.
10. Georgia
Is this illegal? Unclear, but pretty much yes.
Ga. Code § 21-2-413 states, "No elector shall use photographic or other electronic monitoring or recording devices or cellular telephones while such elector is within the enclosed space in a polling place." (Emphasis added.) Ga. Code § 21-2-2(27) defines the "polling place" as "the room provided in each precinct for voting[,]" while Ga. Code § 21-2-267 indicates that the "enclosed space" is the area within a "guardrail or barrier closing the inner portion of such room, which guardrail or barrier shall be so constructed and placed that only such persons as are inside such rail or barrier can approach within six feet of the ballot box." It is not clear whether use of recording devices may therefore be permitted within the "polling place" but outside of the "enclosed space"; check with local officials before taking pictures or video.
A big thanks to Citizen Media Law Project, for comprehensively organizing the laws, state-by-state. See the full chart here:
11. BONUS:
Some Instagram users are trying to spread around photos of ballots to lose votes for candidates they don't like. (This seems pretty unlikely to work.)