There are two sections to the poem "This is a Photograph of Me." The first section describes an old photograph depicting a lake, some hills, a house, and the branch of a tree. If we only had this section of the poem (and the title), we would assume that the speaker ("me") was in the picture, perhaps posing in front of the lake or the house. We might then suppose her appearance juxtaposed to this natural, serene setting.
The second section of the poem is in parentheses. Writers use parentheses to add supplementary material or critical commentary. In other words, information in parentheses is not always necessary to get the meaning of the main text. Therefore, the information that the speaker has drowned does not really prevent the reader from trying to picture the speaker, as the reader attempts in the first section. It certainly does add a shocking insight, but remember that in the first section, we had only the physical description of the environment; not the speaker herself. Therefore, we had to conjure her. Consider these lines:
I am in the lake, in the center
of the picture, just under the surface.
. . .
but if you look long enough,
you will be able to see me.)
Whether or not we know the speaker is dead, we still can not see her. We have to look beyond the surface of the photograph and/or beyond the surface of the lake. Maybe this is a statement about looking beyond the superficial. Maybe she haunts this place or the memory of this place haunts her. One thing is for sure; the advice the speaker gives is that one must look past the surface of things (and/or people) to truly understand them. If there is a mystical quality here, it might be that a deeper, more profound understanding of a poem, of life, or of another person is analogous to looking at a gravestone (the lake or photograph being like a gravestone) and that such a gravestone is only a surface behind which lies memories and an entire life story. To discover the deeper meanings and stories, you have to "look long enough."