You could create a quick Photoshop action that (just have Photoshop trace your actions) does a Save for Web and then closes the file. Then just drag the 50 files in and hit the shortcut key you assign to the action 50 times.
Obviously you could create a more complex action (one that repeats itself, for example), but since you're only doing this the one time a quick action would do the trick. The whole thing should take about 5 to 10 minutes.