Using the Image Processing Toolbox is the easiest way to get up and running. If you opted for MATLAB because of its rapid prototyping qualities and relatively quick to learn language, then continueing that path by enhancing the basic MATLAB by purchasing also the Image Processing Toolbox would make much sense, especially because the documentation coming with it is quite useful, I may say essential for a beginner.
There are indeed some third party toolboxes available, which could help you out. Search for instance for IMAT or PIOTR and see if they would help you to get started. If you would find others, please let me know, I am myself also much interested in finding such toolboxes !!
There is also the DIPimage toolbox, which for me is an excellent tool for in general learning about image processing. But as DIPimage also uses some own ideas, you have to carefully cross read between the DIPimage and the Image Processing Toolbox worlds in order to get them well combined. I never made it to get into contact with the DIPimage developers, so take the following statements of mine about their philosophy with caution, it is just my personal impression: DIPimage has its purpose to finally guide you to programming also in other languages, while MATLAB is "only" an educational starting point into the topic of image processing for the DIPimage developer team, and for compatibility reasons DIPimage then does not excatly use the data containers for storing your images as pure MATLAB would suggest to use them. So, carefully study the differences between the DIPimage and the Image Processing Toolbox worlds, if you want to to use them side by side. However, DIPimage is a great tool, and you should check it out!
You might also want to search by the key words "computer vision" and find several packages about it, which often include also the more basic image processing functions and do not all depend on the Image Processing Toolbox as a base package.
Finally, you could learn how to incoorporate functions from other language´s function libraries into your MATLAB project. There are many great image processing function libraries available, which have been written for instance in C or C++, and are freely availabe. Maybe you stay with MATLAB as your rapid prototyping environment, but make use of the fast image processing functions readily available out there in the internet? Get an idea about this by checking out what for instance the ITK or OpenCV packages are.
However, I enjoy learning with the Image Processing Toolbox how to process images and would not want to go without it. I am just afraid that my limited budget might block my acces to an up-to-date version of it, some day in the future. The newest MATLAB version only runs the newest toolbox versions, you cannot update only one of them according to your needs; you would have to update all of them synchronously - and pay always for all of them as your accumulated software package! Therefore I from the beginning on need to care to in general not become too dependent on too many original toolboxes, especially if MATLAB is not the core tool of my daily work in a life science laboratory and thus one of the first tools my limited budget could force me to downsize, in the future.
