I'm very new to cryptography (and cryptanalysis), and I'm wondering how to decode a substitution cipher, where all of the letters are replaced with symbols, that is, pictogram cipher?
There is little difference with regards to any attack method that is already available for substitution ciphers. You can do frequency analysis, for instance. If a single symbol is used for the letter e or for space then it should be easy to find.
It's easy to see that there is no fundamental difference: just replace each symbol with a character and you're back to a normal substitution cipher.
If there are symbols that represent sets of characters or if there are more symbols for the same character then the cipher will of course be harder to crack.