Pictures, Sound, Video with App Inventor
App Inventor can do lots with media. Here are some hints and suggestions to ease your learning.
To upload video and sound into App Inventor you need clips that are less than 1Meg. You cannot have too many of them in one app as the total size of you app is limited to about 5Meg.
For sound, small .mp3 files are best with the Sound component. Large files can be used with Player.
I find that .mp4 and .3gp video files work well. Both are H264 encodings of video with is generally supported on all Androids.
A source for short .mp3 sound files is http://www.free-ringtones.cc/
One source for videos is http://www.4shared.com/
Want the full story on supported media files? See
http://developer.android.com/guide/appendix/media-formats.html