Image Sampler not sampling correctly

I'm having an issue using the image sampler.

I am trying to sample in a low-res greyscale image and pull out the value of each pixel.

I started with an image 100x25 but when I sampled I kept coming up with duplicated rows. When I set the domain to X= 1 to 100 and Y= 1 to 25, the image would be stretched and I would only see the bottom left corner of the image. (Some rows and columns were duplicated along the way).

Now I've tried to simplify the definition and try again with a 5x5 image and a 10x10 checkerboard test image. Now I'm not getting any variation, all the values are the same!

I hope I've merely done a stupid mistake along the way, and its not that the image sampler is broken.

Basically I want to create a grid of rectangles, find the center point and sample the image's brightness. I would (eventually) like to operate on this data. For now I am testing the data by visualizing using the preview geometry component.

Please help!