Anything binary can be converted (reversibly) to ASCII by the Base64 encoding. One of the libraries that implements it is Jakarta Commons Codec. Understand, though, that no image-specific operations can be performed on that ASCII representation until it is decoded back to binary. [ January 04, 2007: Message edited by: Ulf Dittmer ]
Base64 encoding (and decoding) is actually built into the JVM core classes -- via the sun.misc.BASE64Encoder and sun.misc.BASE64Decoder classes.
Interestingly, these classes have come into "common" usage to the point that they are also built into the IBM JVM as well. (with the same package and class names)