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]];