The png format is not well supported at this time. It is too new for good compatibility.
Other things you might check:
- Check for a typogoofical error.
- Validate your code with W3C. Bad code throws browsers into quirks mode.
- Is the cache too small for the file?
- Is the image too large for the space it needs to render in?
- You might need a plugin.
- Is the CPU is too busy doing something else?
- Maybe the ISP can't or won't serve the file type. It is used for pirate videos.
- Older browsers and servers don't know what .png files are, or can't render animated ones.