Can I use the image texture to select vertices?
do you mean like this?
Note: depending on your image (size) you may want to change the scale and add node values so that you can see "something".
Also it might be necessary to stretch the position values a bit so that the image won't get distorted (you can do this with a vector math multiply node)
..and of course you could use "more intelligent" ways to determine whether the instance should be shown or not than just compare alpha with greater than. You could e.g. take just the red value of the color value of the image texture...or whatever
unfortunately there is no "automated" way up to now to get the size of an image to adapt the size of the grid proportionally.