I think that this would turn out to be an interesting task. I would develop the comic strip in a frame format. This would allow you to be able to show the incremental nature that is such a strong part of the poem. A frame approach would be able to show how the frog manipulates the nightingale into ruining her voice, and how the frog benefits from this. It would also allow you the chance to inject your own opinions about how who is more at fault in the poem. Is the frog to blame because he is able to exercise his influence over the nightingale or is the nightingale to blame as she allows the frog to hold such sway over her? I think that both could be shown quite nicely in a frame by frame analysis of the poem. Your opening frame could consist of the frog sitting by himself in the bog, croaking out his sounds to the relative dismay of the others around him. At same time, the frame approach allows you to develop captions for each drawing, letting the reader appreciate what is happening while enabling you the chance to inject your own perspective on the events transpiring. Adding your narrative voice to the events of the poem unfolding through a frame approach will allow the comic strip form to capture the essence of the poem.
To create a comic strip based on the poem "The Frog and the Nightingale," you might want to start with about eight to ten important moments in the poem, each of which could be turned into a frame. For example, you might choose the frog croaking away in the bog. You might also choose the nightingale beginning to sing and being applauded by the other creatures in the bog. For each frame, which represents an important moment in the story, sketch the characters and a speech bubble representing what they will say. Then, draw in the background of each frame and add text in each speech bubble. You can paraphrase the lines of the poem rather than reproducing it verbatim in the speech bubbles. For example, when the frog praises the nightingale's singing, you might include the word "bravo" in the speech bubble rather than including all of the lines in the poem. In general, you want to keep the speech bubbles short and clear. Finally, you should add color and detail to each frame.