I am trying to get a user's thumbnail from another intranet site but some of them do not follow the pre-defined format meaning I would want to load up a default thumbnail instead.
Whats the best way to check if an image URL is valid?
Depending on how you are getting your images a variation of this might work
<html>
<body>
<img src="<dynamic handler url>" alt="My Username" onError="this.src='defaultProfile.jpg';" />
</body>
</html>
This is how you would do it in ASP.NET.
Designer -
<asp:Image ImageUrl="NonexistentImage.Jpg" ID="profileImage" Height="50" Width="50" runat=server />
Code Behind (c#)
profileImage.Attributes["onerror"] = "this.src='http://www.cs.uofs.edu/~olivetoj2/blah.jpg';";
This works perfectly for me.