It is clearly a bug in the instructions plus a confusing UI
The "load from image" refers to the image actually selected in the editor, it its not loaded from a file.
From the screenshot you send, it seems you try to load the main image as a threshold. The width and height should be 8 if you load the "half8x8o.png" threshold image and you get a 2136 x 3216 user threshold.
To load a pattern from an image you must select the threshold image (half8x8o.png) as the selected image in the editor before you start the plugin, then load this image as threshold, and finally close the plugin to select another image (to which you want to apply the dither)
There are two things in the UI which could be confusing, so I published another version (1.21) with the following changes.
1.- The input field "File" should be disabled when the radio button "From actual image" is selected.
2.- After the threshold pattern image was loaded, it don't make sense to apply it to itself, and the user can't select another image without ending the plugin.So the plugin will automatically end to allow to select another image in the editor.
I hope now it works
It will be more intuitive to load the image from a file, but there is a reason to do things this way.
To load an image from a file, the plugin should use an external library instead of the image editor to get the image.
Adding another library, to do something the image editor could do seems a very bad idea.
Regards
PS last version 1.22. Due to windows registry limits, the threshold size could not be greater than 262144 (width*height)