Removed notes I wrote to myself and shouldn't be there in the previous commit. Realigned method declaration.
Indeed, as pointed by Роман Донченко, m_block_size does not need to be of size_t type. On the other hand all tests I made showed that the image only loads correctly with m_block_pos being of type size_t. I could not figure out if there is some other change in the code that can be done to avoid changing the the type of m_block_size.