When upgrade wkhtmltopdf to 0.12.6, it came to such messages and the image did not show in the target pdf:
Warning: Blocked acc
For the C API, contrary to what the documentation says, it's not load.blockLocalFileAccess but loadPage.blockLocalFileAccess that you must set to "false":
wkhtmltoimage_set_global_setting(settings, "loadPage.blockLocalFileAccess", "false");
Hopefully, the documentation will be updated soon; see issue #4763.