Regarding step 6 of the instructions:
"6) Add the .js file as a web resource in AssemblyInfo.cs of SitefinityWebApp:
Is this right? Don't you mean in AssmeblyInfo.cs of the ThumbnailSelectorField project?
Also step 7, registering a virtual path
- this is what the code in global.asax does, no? Is there a need to manually register it through the backend?
I downloaded your supplied code and have followed the instructions in your blog post, and I've ended up with the same error I had before. "Value cannot be null. Parameter name: stream". I've made no changes to the code and have double checked the installation instructions.