Added convert binary detection

Qalbum.py

@@ -409,6 +409,23 @@ def natural_sort_key(s, _nsre=re.compile('([0-9]+)')):
     return [int(text) if text.isdigit() else text.lower()
             for text in re.split(_nsre, s)]    
 
+def which(program):
+    ''' emulate shell which command '''
+    def is_exe(fpath):
+        return os.path.isfile(fpath) and os.access(fpath, os.X_OK)
+
+    fpath, fname = os.path.split(program)
+    if fpath:
+        if is_exe(program):
+            return program
+    else:
+        for path in os.environ["PATH"].split(os.pathsep):
+            exe_file = os.path.join(path, program)
+            if is_exe(exe_file):
+                return exe_file
+
+    return None
+
 def traverse(path,crumbs,inputs,options):
     ''' The recursive main function to create the index.html and seek sub folders '''
     
@@ -509,6 +526,9 @@ def setupoptions():
     
 def setupdefaultoptions(options):
     ''' Adds the missing options for the options object '''
+    if not which('convert'):
+        print('You don\'t seem to have ImageMagick "convert" in PATH!')
+        sys.exit(1)
     if 'attachments' not in options:
         options.attachments=True
     if 'clean' not in options: