When you say "visual materials" I'm going to assume you mean non-text documentation.
At this state of your project, you need to communicate 3 things to the stakeholders: establish the need, quantify the benefits of meeting that need, and scope the solution. It sounds like you may not be able to scope the solution sufficiently without funding research and/or testing, so you may need to break up your solution into a specific scope for research and a ballpark for the overall solution.
As for the visual part, I would focus on graphics that represent the problem's evidence metrics (if you have them) or heuristic evaluation of the existing system (if you don't have metrics), and the expected magnitude of the impact of solving the problem (e.g. a graph of potential sales lost). If you have realistic data about the potential gains, then incorporate a nice bar graph scaled so that the relative improvement looks impressive.
I would steer away from mockups of architectures or UI for the proposed solution, unless you are confident that you already know everything you need to properly solve the problem. These tend to lock in expectations and make later changes more difficult. They can also trivialize the effort required in the minds of the stakeholders -- Why am I paying you for research & design, just build what you showed me! --
This is a general starting point based on my own experience - best would be to get insight into the boss' mindset and tailor your pitch accordingly.