diff --git a/src/mcnoaa_tides/tools/charts.py b/src/mcnoaa_tides/tools/charts.py
index be36fd9..b5775aa 100644
--- a/src/mcnoaa_tides/tools/charts.py
+++ b/src/mcnoaa_tides/tools/charts.py
@@ -92,7 +92,7 @@ def register(mcp: FastMCP) -> None:
             from mcnoaa_tides.charts.tides import render_tide_chart_png
 
             png_bytes = render_tide_chart_png(predictions, observed, station_name)
-            return Image(data=png_bytes, format="image/png")
+            return Image(data=png_bytes, format="png")
         else:
             from mcnoaa_tides.charts.tides import render_tide_chart_html
 
@@ -179,7 +179,7 @@ def register(mcp: FastMCP) -> None:
             from mcnoaa_tides.charts.conditions import render_conditions_png
 
             png_bytes = render_conditions_png(snapshot, station_name)
-            return Image(data=png_bytes, format="image/png")
+            return Image(data=png_bytes, format="png")
         else:
             from mcnoaa_tides.charts.conditions import render_conditions_html