I think the simple present is surely the default.
I wonder if it depends partly on whether the cartoon is a joke with a punchline. It seems to me that the simple present is the normal tense for jokes: A minister, a priest, and a rabbi walk into a bar. The minister says ... And the priest says ... And then the rabbi says ... So to me, if the cartoon is a joke, you would tell it in that narrative way.
In a classroom exercise, if the teacher's question is 'What is happening in this frame/picture,' the students might say 'In the first frame the minister, the priest, and the rabbi are walking into a bar.' But that still sounds wrong to me if the picture doesn't specifically show action in progress, e.g., legs moving, the door of the bar opening, people going through it. And in a still picture such as a cartoon, I wouldn't really expect that; it's more a snapshot than an action.
So in general, I would avoid the progressive in describing comic strips or cartoons unless there is visible action in progress. And to me, speech doesn't count as action in the sense of movement.
Just as another tip, I would never write 'In the speech bubble.' It's not necessary to draw attention to the fact that cartoons use bubbles or other conventions to represent speech; everyone knows that. Just write something like
In the first frame/picture, the man is speaking to his wife. He says, 'Honey, I'm home.'
It's essentially like any other narration, telling a story; books don't use the progressive for reported speech.
However, for an indirect quote you could probably use the progressive:
In the first frame/picture, the man is speaking to his wife. He's telling her that he's home.