Update test_image to use SI units and long aliases

Author: weiji14

pygmt/tests/baseline/test_image.png.dvc

@@ -1,4 +1,4 @@
 outs:
-- md5: ffb11c7f12025471b2556c1473cd3000
-  size: 13973
+- md5: de86468aa453b14912c8362c67e51064
+  size: 10403
   path: test_image.png

pygmt/tests/test_image.py

@@ -17,5 +17,5 @@ def test_image():
     Place images on map.
     """
     fig = Figure()
-    fig.image(TEST_IMG, D="x0/0+w1i", F="+pthin,blue")
+    fig.image(TEST_IMG, position="x0/0+w2c", box="+pthin,blue")
     return fig