Changed the upload_to of item pictures to a static functions, as lambdas

do not work in migrations

src/pretix/base/migrations/0006_auto_20150726_1420.py

@@ -0,0 +1,20 @@
+# -*- coding: utf-8 -*-
+from __future__ import unicode_literals
+
+from django.db import models, migrations
+import pretix.base.models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('pretixbase', '0005_item_picture'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='item',
+            name='picture',
+            field=models.ImageField(upload_to=pretix.base.models.itempicture_upload_to, null=True, verbose_name='Product picture', blank=True),
+        ),
+    ]