AFAIK that isn't supported. You can 1) embed an image directly in the report, 2) use a relative path to an image that is stored in the SSRS catalog, use a URL or retrieve the image from a database column.
Note that mapped drives are per user so you cannot use a mapped drive anyway. You can try using the full UNC path instead (e.g. \\server\share) and see if it works. However be aware that support for this has become questionable for security reasons and some browsers may not support it. Using the file:// URL is also potentially limited. In all cases SSRS needs to have access to the file on the remote path in order for it to render anything.