First off, don't just concatenate strings to form the path: unless the user remembers the trailing '/' the URL will be significantly different and will fail.
Use
Path.Combine[
^] instead.
Second, we can't check what is happening at all, as the site that the image is being sourced from depends on what the user enters, and we have no access to that.
So, it's going to be up to you.
Fortunately, you have a tool available to you which will help you find out what is going on: the debugger. If you don't know how to use it then a quick Google for "Visual Studio debugger" should give you the info you need.
Put a breakpoint on the first line in the function, and run your code through the debugger. Then look at your code, and at your data and work out what should happen manually. Then single step each line checking that what you expected to happen is exactly what did. When it isn't, that's when you have a problem, and you can back-track (or run it again and look more closely) to find out why.
I'd start by looking at the completed URL closely, and then instead of building an image directly, I'd read the stream into a byte array and have a good look at that. Chances are that either the URL is wrong, or the site is returning an error message instead of the expected image stream - but only you can tell that via a close look using the debugger.
Sorry, but we can't do that for you - time for you to learn a new (and very, very useful) skill: debugging!