Replace `std::Vec<RgbImage' with `std::Collections::HashMap' for cleaner data handling

In the first attempt, `std::Vec was used to process the images. This data structure doesn't contain the titles of the images that need to be processed by the program. std::collections::HashMap<String,RgbImage> can contain the image titles in it's key and the processable image data in it's value. All the appropriate methods must be applied to this data structure.