*.tsc files. They can be edited using either Sue's Workshop or Cave Edit. Both of these can be found on the Tribute Site (cavestory.org). You'd also have to edit "graphic text" (i.e. the word "WARP" over the teleport window, the "ARMS" and "ITEM" text in the inventory, etc) which is in the *.pbm files, which are secretly just *.bmp files in disguise (meaning that you can edit them with any software that edits *.bmp files).