How do I mass convert AVI files in my iPhoto library to MPEG?

You can download a video converter to do that. I used many video converters. "Format factory" is a free video converter, and it is very useful. And If you want to find a professional video converter, Aimersoft Video Converter Ultimate is a powerful software, which can easily convert video file and DVD, burn video to DVD, even download online videos Here you can download it: http://www.goshareware.com/video-converter-ultimate.html

I highly recommend you to use a Totally FREE but powerful third-party video converter called Total Video Converter Lite. I have used this app several years and it is really nice! This app supports to convert AVI files to MP4. https://itunes.apple.com/us/app/total-video-converter-lite/id520374433?mt=12 The upgraded version Total Video Converter Pro supports to batch convert many video files. https://itunes.apple.com/us/app/total-video-converter-pro/id426654691?mt=12 Total Video Converter Lite, a totally FREE but most powerful converter, handles HUNDREDS of video and audio types to Apple TV, iPad, iPhone, iPod, DVD, Xbox 360, PS3, PSP, Blackberry, Nokia, Samsung ...  Total Video Converter Lite is also able to convert AVCHD ( *.M2TS, *.MTS, *.TOD ), HD MKV, HD H.264 / MPEG-4 AVC, HD MPEG-2, HD MPEG Transport Stream ( *.TS, *.TP, *.M2T ) and other high-definition video formats to normally used SD / HD video formats. Also, you can convert SD video formats to HD video formats including AVI, WMV, MP4, MKV,FLV and etc. Auto Import HD videos from your HD camcorder and camera. No technical knowledge is required. Simply drag your file to Total Video Converter Lite, select an output format and convert, it makes video conversion quick and easy.

AVI, abbreviation of Audio Video Interactive, is basically a multimedia container format that was first released by Microsoft in November 1992. It enjoys great fame for its great flexibility and compatibility with a large number of devices and media players. But high versatility does mean omnipotence. Compared with AVI, the major advantage of MPEG file is that it is much smaller for the same quality, supported by Apple products and can be recognized by nearly all DVD players. That is why many Mac users want to convert AVI to MPEG format. Disappointingly, such freeware usually has problems of either disappointed quality or slow conversion speed. Worse still, some free AVI to MPEG converter crashes on Mac computer. But if you are still racking your brains in trying to free convert AVI to MPEG on Mac, you can make all of your worries pass away like a cloud with the assistance of http://www.ifastime.com/mac-video-converter-ultimate. This is a simple program that specializes in digital video conversion including .avi format from iPhone to MPEG, it's easy to use and supports conversions between virtually all popular video & audio formats. If you are looking for a way to make reverse conversion from AVI to MPEG file on Mac OS X, the program can do it, too. Besides, it also comes in handy for frame cropping, movie trimming, adding subtitles and video effect adjustment, etc. Note the program is available for Mac, you can get its counterpart - http://www.ifastime.com/video-converter-ultimate to handles your AVI files on PC.