Unfortunately, most buttons in the button in the toolbar don't have unique class names.
You could target using nth-child()
but that's risky, as any extension install or update might change the order.
If you know which language(s) will be used on the forum, the best is probably to filter on the text of the label, like .MarkdownToolbar .Button[aria-label="Add an image"]
I see Anomalum beat me to the reply. I'm still posting as I'd recommend having .MarkdownToolbar
in the selector, to make sure the code does not affect other unrelated elements of the Flarum interface or in third-party extensions.