Image Sampler - getting started

Hello, 
I’v just begun learning grasshopper. I'm using Rhino4 SR8, grasshopper 0.8 and I’m facing a problem with the Image sampler tool. Every time I select the Image sampler or simply drag and drop an image into the grasshopper file, I get a warning that states ‘floating parameter failed to collect data’. 
Could someone tell me what that means and how I can get my Image sampler working?