The first one you might recognize by the artifacts at the four corners of the image, they suggest that something funny is going on in the frequency space. In other words the classic way to find the hidden message is to perform a 2D fourier transform on the image. Perhaps the simplest way to reveal the message is to use a gimp plugin (for example G'MIC). You can also make these images using G'MIC and simply painting your hidden messages and then inverse transforming them. However, I did mine in matlab. In this case I transform the red color component.
pic1 = imread('d:\babel\20131215\me.jpg');
imagesc(log10(abs(fft2(pic1(:, :, 1)))), [0 5]);
The image suffers a little bit from jpg-compression, but no worries. The logarithmic scaling is unimportant, you see it just fine even with gimps default settings. You could use the same idea to encode messages into mp3s for example and they would be revealed in their spectrogram (4:10 is interesting in this video).
Perhaps the most famous was the face used by Aphex Twin at the end of one of their tracks (at 5:30).
Here's how I did my picture:
pic = imread('d:\babel\20131209\me.png');
MSG = imread('d:\babel\20131209\msg.png');
MSG = imresize(MSG, [406 640]);
R = fft2(pic(:, :, 1));
R = R + 15*double(MSG);
r = ifft2(R, 'symmetric');
pic(:, :, 1) = abs(r);
imwrite(pic, 'd:\babel\20131209\me.jpg', 'JPG');
You could do something similar even for a video.
The second image is a bit more interesting because no visible signs can be seen and it would be very difficult to recognize anything but noise even if you could compare it to the original unaltered image. Yet the method is fairly simple. The least significant bit of every pixel is part of 8-bit ASCII encoded message (notice how the image is encoded using a bit perfect PNG-compression unlike the first image).
pic2 = imread('d:\babel\20131215\cat.png');
sprintf(char(bin2dec(vec2mat(sprintf('%d', double(mod(pic2(1:1702*8), 2))), 8))))
Don't pay too much attention to the trickery in the code. There are numerous ways to do it all equally well. You could even encode the message into a lesser number of bits like 7 as ASCII is often encoded in or you could even make your own character sets or include further tricks, however I figured that doing so would make it unnecessarily hard.
And here's the code to make the image with the message:
message_in_bin = dec2bin(message, 8);
A = imread('d:\babel\20131209\org_cat.png');
ss = size(message_in_bin);
message = message_in_bin';
for x = 1:ss(1)*ss(2)
value = bin2dec(message(x));
if mod(A(x), 2) ~= value
if A(x) > 100
A(x) = A(x) - 1;
A(x) = A(x) + 1;
imwrite(A, 'd:\babel\20131209\cat.png', 'PNG');