One of the best tools I have found for dealing with new media files (images, audio, and video formats) is Shutter Encoder.

I have used this program to convert transparent video files to WebM format keeping the alpha intact, to resize files, and do mininal video edits as well.  

It also handles audio formats and Images. 

I was looking for a way to convert over 100 image files from TIFF and JPG to PNG + get them all smaller sizes appropriate for a website I built for an artist.  http://tonypetersart.com/

He gave me over 100 files in different formats and different sizes that needed to be converted quickly and easily.  Shutter encode has an IMAGE option that let me select all the files and convert them at once!

This program will save you so much time! 

I haven’t been paid, nor asked to make this post, I just wanted to share this information because I couldn’t find much help on reddit or in my google searches.  I’m hoping this post gets picked up when people are searching for ways to batch convert images, or convert MOV files to WEBM with Alpha Transparency.