Save as PNG grainy

[robposs]

Using Photoshop 6 when I save as a PNG file they appear grainy in other programs eg bridge and the Vistaprint photobook program. They are fine as PNG within photoshop but not outside Photoshop. Other files TIFF PSD PSB  CR2 are all fine but unfortunately Vistaprint doesn't accept TIFF(it used to) or PSD so I need sharp PNG files.

Is there some way to save them smooth and sharp as PNG

robposs

[c.pfaffenbichler]

Can you post one such image?

Have you tried Save for Web?

[robposs]

The files are large too large to save for web. They are for printing. I tried a reduced size sample but can't reproduce it at smaller size.

Does PNG have a problem with large files?