macos: reduce some vpn screenshot sizes

osx/vpn_mac.markdown

@@ -13,7 +13,7 @@ Click the `Download for macOS` button to download the Cisco installer and run it
 
 After the installation, you may get a notification, that the system extension by Cisco was blocked.
 
-[[!img /media/osx/vpn_mac_2.png size="450x"]]
+[[!img /media/osx/vpn_mac_2.png size="250x"]]
 
 Open the System Preferences, and manually allow the Cisco Socket Filter
 
@@ -23,7 +23,7 @@ Open your Applications folder, navigate to the Cisco subfolder, and open the `Ci
 
 When prompted, allow the Cisco software to filter your network traffic.
 
-[[!img /media/osx/vpn_mac_4.png size="450x"]]
+[[!img /media/osx/vpn_mac_4.png size="250x"]]
 
 Click on the `Connect` button and enter your VPN credentials, including the `@staff-net.ethz.ch` realm for the username, as shown below.