diff --git a/regtest.py b/regtest.py
index 954ceab..2ab62fb 100755
--- a/regtest.py
+++ b/regtest.py
@@ -1058,7 +1058,9 @@ def test_suite(argv):
                             ppm_file = test_util.get_recent_filename(output_dir, "", ".ppm")
                             if not ppm_file is None:
                                 png_file = ppm_file.replace(".ppm", ".png")
-                                test_util.run(f"convert {ppm_file} {png_file}")
+                                from PIL import Image
+                                with Image.open(ppm_file) as im:
+                                    im.save(png_file)
                                 test.png_file = png_file
 
                     # analysis
diff --git a/requirements.txt b/requirements.txt
index 8ce531d..bf24963 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -2,3 +2,4 @@ matplotlib>=2.1.0
 sphinx_rtd_theme
 Sphinx
 nbsphinx>=0.3.1
+pillow