In reply to Dev_Engine:
There are some inconsistencies in your question, so some clarification is needed.
The string data type is an ordered collection of characters. You state that your data is "amb124hk2k3b2jb42223fmf...", which would be a single string. An queue of strings would be multiple strings, not individual characters.
You don't show the output of your module 'image_tests', so it is difficult to determine if your connections are correct.
The find_index() function will not search each string. Searching for "b", or any other single character, won't work. The function will only match the entire string.
Please check your data types and provide clarifying information.