Odd graphics error -- "division by 0"

The unable to load picture or PDF file error is a clue that something's wrong, not on the TeX side but on the filesystem side. Possible causes include a corrupted image file, a permissions-protected image file, or a nonexistent image file. The division by 0 error is secondary to the first: TeX is trying to draw a box without having dimensions to form its corners.

If you can open and view the image with a previewer then it's likely the problem is not with the file itself (or its permissions, since TeX is running as you). So I guessed the nonexistent file, i.e., bad file name. Knowing that Dropbox usually doesn't mount at the top level of the file system was another clue that the path was wrong.

cfr's check that this is the problem is worth repeating.

copy the image file to the working directory, delete the leading parts of the path from the filename in the source, and try compiling.

It's another example of how simplifying the non-compiling document down to the barest bones can isolate the error.