// prefix is Blender specific to denote relative paths used within Blender,
To use Python's filesystem functions you'll have to first expand this using:
filepath_full = bpy.path.abspath(image.filepath, library=image.library)
./ however, which simply expands the current-working-directory (
os.getcwd()), Blender supports library-linked files. This means the directory of the current data-block could be a different path then the currently open Blend file.
If you are operating data which the script created, You can skip passing the
filepath_full = bpy.path.abspath(image.filepath)
Note that this isn't specific to images, it works for movie-clip, sounds, sequencer and text blocks too.