Learn English – When people say “I’m twelve years old in this photo”, does it have any non literal meaning

Max has covered most of the answer well, so I will just offer an alternative interpretation of the final sentence.

As this is essentially speech, not prose, you can't assume proper use of grammar. An alternative reading of the final sentence is therefore "Fuck this, nigga.", where "this" refers to the instructions from "D", not "D" the person, and "nigga" is the person they are talking to. In this interpretation, it implies that the person saying this is going to paint the room blue despite D's advice.

In the interpretation proposed by Max, the sentence splits up in to "Fuck" and "this nigga". This is just an exclamation of contempt for "D" and doesn't necessarily imply any action from the speaker.

Further context may help you decide which meaning is more relevant. I would argue that the exact meaning as posted is ambiguous, but essentially amounts to the same thing: the speaker is saying that they are unhappy with being told they can't paint the room blue because Tamiya is a girl.

I would like to echo others by strongly advising that you don't use the word "nigga" as it is very likely to cause extreme offence to others. I would recommend never using swear words in general in any polite context, regardless of what other people are doing, and to only use them around others in an informal setting if they start using them first.