^ exactly.
now that you know what texture you want to edit, open the map in HMT and click the bitm tags carrot.
highlight that bitmap, hit the "preview" button, and then the "export" button.
save the file somewhere and edit it with a photo-editing application (like photoshop or gimp).
when your bitmap is ready, hit the "inject" button. a window will come out.
drag the new bitmap inside that window, and click "inject".
after a minute or so, HMT will crash. it means that the bitmap is imported properly.
...for a better explanation, go
here.
(i really must finish that tutorial...)