diff --git a/.gitignore b/.gitignore
index 8e4864f8b5d8cf5b96c5953f5aa2767c234e88b0..b2ed3cb14138de4545d6668efb81895c693b3669 100644
--- a/.gitignore
+++ b/.gitignore
@@ -9,7 +9,6 @@ results/
 *.egg
 *.log
 *.out
-*.png
 *.pdf
 *.jpg
 *.jpeg
diff --git a/doc/spectrum.png b/doc/spectrum.png
new file mode 100644
index 0000000000000000000000000000000000000000..e5168435ce2d267ccc0bd5bcc006d770a6c06959
Binary files /dev/null and b/doc/spectrum.png differ
diff --git a/doc/u.png b/doc/u.png
new file mode 100644
index 0000000000000000000000000000000000000000..b49b9181851a530e5cbf29f70dc4132d20769ee2
Binary files /dev/null and b/doc/u.png differ