diff --git a/USAGE.md b/USAGE.md
index 22420d4..f5a234f 100644
--- a/USAGE.md
+++ b/USAGE.md
@@ -12,7 +12,7 @@ I personally use the wand selection tool with anti-aliasing turned off to select
 To expand selections in Photoshop, do Selection > Modify > Expand or Contract.
 To expand selections in GIMP, do Select > Grow.
 
-Save these images in the PNG format to the "decensor_input" folder.
+Save these images in the PNG format to the "decensor_input" folder. **They MUST be in PNG format.**
 
 ### A. Using the binary
 
@@ -52,4 +52,4 @@ Decensored images will be saved to the "decensor_output" folder. Decensoring tak
 
 ## III. Decensoring with the user interface
 
-To be implemented.
\ No newline at end of file
+To be implemented.