I just did a test. If I click on a picture in iPhoto and select ‘email’ it creates a standard jpeg in one of three sizes of my choice.
This is the usual and easy way to email photos. If she is attaching a picture from another source or another format that could be it.
In general, Macs read almost all formats, including Window specific ones like bmp. And they can write and save pretty much every format. I’d check to make sure she’s sending jpgs to you, and again the easiest way is to select them in iPhoto and click Email. It will create the jpgs and attach them to an email.
Her email program must obviously change the format to something she can see when they come in....but doesn’t change them BACK when she forwards them. That’s pretty d@mn inconsiderate ;)