This tutorial shows what type of image, sound and movie formats can be used in x3dom and what are the features and restrictions.
You can use PNG, JPEG or GIF to encode your static Texture data. JPG has a low memory profile but has a lossy compression and it does not support alpha channels. PNG compression is lossless and can handle alpha. GIF is also lossless and has alpha.
General: If you do not need an alpha channel and the content does not have hard edges (e.g. Text) use JPG. Otherwise use PNG. You should really not use GIF anymore. PNG is more flexible for future content (e.g. 16-bit channels).
<ImageTexture url='foo.jpg' />
<AudioClip url='"foo.wav","foo.ogg"' />
There is right now no single movie file supported by all UA. Use this link to check your browser.
<MovieTexture url='"foo.mp4","foo.ogv"' />