Specify absolute source path and relative destination path using plugin options.

{
	resolve: 'gatsby-plugin-copy-files',
	options: {
		source: `${__dirname}/src/public`,
		destination: ''
	}
},
{
	resolve: 'gatsby-plugin-copy-files',
	options: {
		source: `${__dirname}/src/images`,
		destination: '/images'
	}
}

Must be used with gatsby-source-filesystem.