The ratio 4 to 7 means that for every 4 photos in one group, there are 7 in the other group. So to divide the 33 photos into two groups with the ration 4 to 7 we can make a chart where we would have group one on one side and group two on the other. On one side we would increase by 4 each time and on the other side you would increase by 7 each time. Keep going until you find a row in where the numbers add to 33. So:
Group 1 / Group 2
4 7
8 14
12 21
Here we can see that 12 + 21 = 33 So in one group we will have 12 photos and the other group we will have 21 photos
