dir = getDirectory("Choose a folder");
list = getFileList(dir);
setBatchMode(true);
run("Text Image... ", "open="+dir+list[0]);
run("Copy");
width = getWidth;
height = getHeight;
close();
newImage(dir, "32-bit Black", width, height, list.length);
id = getImageID;
run("Paste");
for (i=1; i<list.length; i++) {
   showProgress(i, list.length);
   run("Text Image... ", "open="+dir+list[i]);
   run("Copy");
   close();
   selectImage(id);
   setSlice(i+1);
   run("Paste");
}
setSlice(1);
resetMinAndMax;
run("Select None");
setBatchMode(false);