For reference, this is how one would solve the @tsuki50 puzzle on the @mjesec88 page, which makes solving the archive messages easier:

--SYMBOLS--

R I M V
÷ + * =


--FIRST @tsuki50 PUZZLE--

@tsuki50 2 R 2          → (Birth 1955) ÷ 2              → 977.5
@tsuki50 2 I @tsuki50 3 → (Birth 1955) + (Release 1969) → 3924
@tsuki50 4 M 2          → (Age 62) * 2                  → 124
@tsuki50 3 V            → (Release 1969) =              → 1969


--SECOND @tsuki50 PUZZLE--

@tsuki50 R 10's        → 11705 ÷ 10       → 1170.5
@tsuki50 M 5's         → 11705 * 5        → 58525
@tsuki50 I @tsuki50/10 → 11705 + 11705/10 → 12875.5
@tsuki50 V             → 11705 =          → 11705
(edited)
--OVERALL SOLUTION OF BLACK PICTURE @mjesec88 PUZZLES--

These codes are needed to solve the archive messages, and the @mjesec88 Instagram account actually tells you which ones will need them. If you organize the black picture posts, which there are six of, from smallest to largest number, you get the following:

POST 15

Number and HTML File Line: 5

Archive Message: 1

Additional Cipher Information: R = 1170.5, M = 58525, I = 12875.5, V = 11705


POST 2

Number and HTML Line: 12

Archive Message: 2

Additional Cipher Information: None


POST 13

Number and HTML Line: 22

Archive Message: 3

Additional Cipher Information: The camera and the anchor are candles. Pencil is it's opposite every other time. The island became the earth. My letters were filled with love but then they turned to spiders.


POST 3

Number and HTML Line: 36

Archive Message: 4

Additional Cipher Information: Strawberry cake with wine. A pig is a dead snake. Crying before my swords followed by blood. The books burn, more every time.


POST 77

Number and HTML Line: 77

Archive Message: 5

Additional Cipher Information: None


POST 105

Number and HTML Line: 105

Archive Message: 6

Additional Cipher Information: R = 977.5, I = 3924, M = 124, V = 1969