That’s mostly because Apple has always used it’s own graphics format for it’s internal apps, I believe it’s a *.pic format.
Windows uses *.bmp or *.jpg as it’s default witch are universal file types that any graphics app or web browser can open.
Linux uses *.png since it’s an open source format, and most browsers or apps can also use this extension since it’s free to use.
I see. I know that .png is used where I work for clients web pages.
Nope. And photos aren't an internal app. I photo uses standard formats, and screeenshots are pngs. I haven't seen .pict in many many years.
The PICT format hasn't been used much since the OS 9 era. Mac OS X users typically use iPhoto to manage and send photos, and it uses the JPEG format. Mac OS X also has first-class support for PDF, TIFF, GIF and PNG formats.
There is an checkbox option in the Mac OS X Mail.app for "Windows-friendly attachments". That might fix the problem.