All the agentic coding models are multimodal and can understand pictures quite well. I routinely paste in primitive paint drawings to augment my textual descriptions and show an agent what I mean, and it seems pretty effective. This can be to rough out a UI layout but it can also be useful in pure backend work drawing diagrams, or in any domain where code manipulates geometric data.

For an example, see my MSPaint drawings in the README (scroll near the bottom) for this tiny little project: https://github.com/rspeele/meshorient

I redrew those for the README but IIRC, I drew something similar when explaining how the feature would work to the agent.

And in reverse, after describing an architecture or an algorithm to it, I'll sometimes ask it to draw a diagram for me to demonstrate its understanding. If it draws the picture in line with what I intended, I conclude that it has gained the necessary context to proceed. If not, I know I explained something wrong or at least insufficiently and need to provide clarification. There are some domains where a picture is worth a thousand words.

It also helps cut through the Claudese. "Your decision is needed for one edge case, found by the gate. When a T-joint meets an endcap that the mesher solves by a fan, should this bisect a fan slice or raise a warning?" I'm sorry Claude, I am not from Missouri but you are going to have to Show Me this one with a picture.

Of course you can save pictures to files and open them with external tools, but it's nicer to have them inlined into the chat history when that's exactly what they are: part of the conversation.