Converting height map bitmap to json
So I would make some changes:

let returnValue = 
    match arglist with
        | path :: _ -&gt; convert path
        | _ -&gt; "Wrong number of arguments supplied"


should probably be something like

let returnValue = 
    match arglist with
        | path :: _ -&gt; convert path
        | _ -&gt;failwith "Wrong number of arguments supplied"


which would simplify some logic

I would also change the printing to use sprintf

"{\"X\": \"" + bitmap.Width.ToString() + 
"\", \"Y\": \"" + bitmap.Width.ToString() + 
"\", \"values\":[" + results + "]}"


becomes

sprintf "{\"X\": \"%i\", \"Y\": \"%i\", \"values\":[%s]}" (bitmap.Width.ToString() ) (bitmap.Width.ToString() ) results