Improve error message (#9392)

Co-authored-by: Andrew Murray <radarhere@users.noreply.github.com>
Co-authored-by: Hugo van Kemenade <1324225+hugovk@users.noreply.github.com>

Author: radarhere

src/PIL/ImageFile.py

@@ -814,7 +814,7 @@ def setimage(
             self.state.ysize = y1 - y0
 
         if self.state.xsize <= 0 or self.state.ysize <= 0:
-            msg = "Size cannot be negative"
+            msg = "Size must be positive"
             raise ValueError(msg)
 
         if (