Hiding and Seeking
               Continuous LSB Embedding

Statistical tests can reveal if an image holds hidden content.

Westfeld and Pfitzmann observe that embedding of encrypted data changes the histogram of color frequencies.
[Westfeld et al, 1999]        
In encrypted data, 0 and 1 bit are equally likely.
If color 2 is more frequent than color 3, inserting encrypted data will change 2 more often to 3, than vice versa.

Same is true for JPEG.  Take DCT coefficients frequency instead.
Frequency difference of adjacent coefficients is reduced.