Aleram with CSS 😄 😅
It would be something like this (paste the code in the "custom CSS" option in the Appearance admin page):
#MarkdownToolbar button[title="Add an image"] {
display: none;
}
#MarkdownToolbar button[data-original-title="Add an image"] {
display: none;
}
If it doesn't work, make sure that the "Add an image" text is the text that is shown when you hover the image button in the toolbar.