Why should a journalist avoid doing things such as altering photographs for dramatic effect?

Because things like that have to show the truth. it is a rule of journalism.

It'd look like they're okay with altering things and fixing up things the way they want it to be - rather than showing everyone the truth and letting them have their own opinion. It shows dishonestly, and it would make people doubt the credibility of their stories as a whole. It makes it seem like they're hiding something.
They do better writing their stories with more of a dramatic effect using words like," And that's not all" or "the next thing that happened shocked millions" etc.
I hope all is well, and you pass! Good luck, rockstar! (:

Conclusion is the answer