The fact that a password is composed of images is irrelevant. If you have 10 possible values for each symbol and choose 4 symbols at random, the keyspace is 104 = 10000, and log2(10000) ≈ 13.3 bits.
This is very insecure!
In general, you can calculate the keyspace by raising the number of possible symbols to the number of randomly-selected symbols. This tells you how many possible combinations there are. You convert this number into the number of bits of security by taking the base-2 logarithm of the result.
For example, if you are choosing a password composed of 12 words chosen at random from a set of 4000, the number of bits of security is log2(400012) ≈ 145 bits, which is secure.