Probably today the more secure algorithm, and the most adopted, for symmetric encryption is AES.
You can use it to encrypt your image (as file stream) in C# and decrypt in Java. Remember that AES is a standard algorithm and the C# and JAVA implementation must be the same. If a encrypted stream is encrypted with a standard symmetric algorithm in one of the two platform, it will be decrypted by the other, due to the standardization of the algorithm.
To quickly encrypt your image with c# you can try Bonnie.NET at
http://www.we-coffee.com/bonnie.aspx. It is a free dll that manages all cryptographic needs when developing with .NET