diff --git a/sigal.py b/sigal.py
index 7339d6e..8967bbf 100755
--- a/sigal.py
+++ b/sigal.py
@@ -46,6 +46,8 @@ def main():
 
     parser.add_option("-c", "--copyright", dest="copyright",
                       help="copyright message added to the images")
+    parser.add_option("-f", "--force", dest="force",
+                      help="force the reprocessing of existing images and thumbnails")
 
     (options, args) = parser.parse_args()
 
@@ -68,7 +70,7 @@ def main():
 
     # create gallery
     gallery = Gallery(params)
-    gallery.build(input_dir, output_dir)
+    gallery.build(input_dir, output_dir, force=options.force)
 
     return 0
 
diff --git a/sigal/image.py b/sigal/image.py
index c51fdd5..9fdc468 100644
--- a/sigal/image.py
+++ b/sigal/image.py
@@ -61,11 +61,12 @@ class Gallery:
             if len(imglist) != 0:
                 yield dirpath, dirnames, imglist
 
-    def build(self, input_dir, output_dir):
+    def build(self, input_dir, output_dir, force=False):
         "create image gallery"
 
         self.input_dir = os.path.abspath(input_dir)
         self.output_dir = os.path.abspath(output_dir)
+        self.force = force
 
         if not os.path.isdir(self.output_dir):
             print "Create output directory %s" % self.output_dir
@@ -106,7 +107,8 @@ class Gallery:
             im_name = os.path.join(img_dir, filename)
             thumb_name = os.path.join(thumb_dir, self.thumb_prefix+filename)
 
-            if os.path.isfile(im_name) and os.path.isfile(thumb_name):
+            if os.path.isfile(im_name) and os.path.isfile(thumb_name) and \
+               not self.force:
                 print "%s exists - skipping" % filename
                 continue