Because I feel that just saying "Use the Search" over and over again isn't a good way of helping people, you can download ResHack here. Open Doukutsu.exe in the program.
To extract the ending images, go to the Bitmaps folder on the left and you'll find folders with each of the ending images. To extract them, right click the entries with the green circles and select the third option, "Extract [BITMAP SOMETHING SOMETHING BLAH]...".
To reinsert the new images, simply do the same but select "Replace Resource", which opens up a dialog to insert the edited BMPs.