Image is the Source
There is no *.markinside file generated alongside when you save images from code. The images are just the single source of truth. You can simply✱ open them in MarkInside to make changes.
✱ Some apps or services may compress your images and wipe away source codes embed inside. You can try to save as SVG or PDF to bypass this issue.
MarkInside offers you a side by side editor, you can see the result as you type or change preferences.
You can adjust windows size and canvas background color, to see how well the output image will suit your use case.
The generated files are just images in PNG, SVG and PDF. Which means you can use them everywhere, without the need for the viewer to support rendering Mermaid, etc.