I've done the opposite before - turning an existing .jpg into an IPicture for embedding in an .mp3 file. I just tried reversing that operation and, after tweaking and testing, came up with this:
TagLib.File tagFile = TagLib.File.Create(mp3FilePath);
MemoryStream ms = new MemoryStream(tagFile.Tag.Pictures[0].Data.Data);
System.Drawing.Image image = System.Drawing.Image.FromStream(ms);
Thanks for the question - I already know how I'm going to use this myself!
Update: Here's the other way (.jpg to IPicture that I've done before):
tagFile.Tag.Pictures = new TagLib.IPicture[]
{
new TagLib.Picture(new TagLib.ByteVector((byte[])new System.Drawing.ImageConverter().ConvertTo(System.Drawing.Image.FromFile(jpgFilePath), typeof(byte[]))))
};