Generate a relative pathname using info script.
Base64 only works with Img both ways. The base64 package gets used as a last resort for SVG generation but cannot being used for Tk-code, because there would be no way to re-code the image that way...
See vgcode.tcl resolveFilePropertyAsBase64 and also svgdumper.tcl.
Example 3.53. ::vgcode::resolveFilePropertyAsFile using rfilepath
...
\
    [string length $projectd] + $idr + 1
}] end];
set value \
    [dumper::rfilepath  \
    [file dirname \
    [vgfile::getName]]  \
    [file split $value]];