The issue is probably the fact that your texture size is set to 2048x2048, which means that it will be scaled down to 2048 if any dimension is bigger. The file you're using has probably 16000 pixels in one of the dimensions :P
Use the sorted files, and they might also be above 2048px, if so change the size of the texture to 4096px.
You'll find the setting in the inspector under the place where you set the filter type to point(no filter).