X_IMAGE_SOURCE_UNREADABLE
Means
a responsiveImage src is not a file the static export holds
Typical cause
responsiveImage({ src }) on a static page named a site path the export does not contain, so no variant of it can be written
Fix
x build --target static --json — lists the export; put the file under apps/web/site/assets/ and pass asset('assets/<file>') as src, or give an absolute https:// URL