Remove short alias from image

Also document the required imagefile parameter that was missing.

pygmt/base_plotting.py

@@ -708,7 +708,7 @@ def logo(self, **kwargs):
  lib.call_module("logo", build_arg_string(kwargs))
 
  @fmt_docstring
- @use_alias(R="region", J="projection")
+ @use_alias(R="region", J="projection", D="position", F="box", M="monochrome")
  @kwargs_to_strings(R="sequence")
  def image(self, imagefile, **kwargs):
  """
@@ -723,17 +723,23 @@ def image(self, imagefile, **kwargs):
 
  Parameters
  ----------
+ imagefile : str
+ This must be an Encapsulated PostScript (EPS) file or a raster
+ image. An EPS file must contain an appropriate BoundingBox. A
+ raster file can have a depth of 1, 8, 24, or 32 bits and is read
+ via GDAL. Note: If GDAL was not configured during GMT installation
+ then only EPS files are supported.
  {J}
  {R}
- D: str
+ position : str
  ``'[g|j|J|n|x]refpoint+rdpi+w[-]width[/height][+jjustify]
  [+nnx[/ny]][+odx[/dy]]'`` Sets reference point on the map for the
  image.
- F : bool or str
+ box : bool or str
  ``'[+cclearances][+gfill][+i[[gap/]pen]][+p[pen]][+r[radius]]
  [+s[[dx/dy/][shade]]]'`` Without further options, draws a
  rectangular border around the image using **MAP_FRAME_PEN**.
- M : bool
+ monochrome : bool
  Convert color image to monochrome grayshades using the (television)
  YIQ-transformation.
  """