go version devel +1be335b Mon Jun 22 11:11:37 2015 +0000 windows/386

package pkg1 // in $GOPATH/pkg1 directory

import _ "image/internal/imageutil"
import _ "image/jpeg"

go build pkg1 command do not occur internal package error.
But if comment out imporot _ "image/jpeg", then occur error.

package pkg1
        imports image/internal/imageutil: use of internal package not allowed

Perhaps, the reason is that there is import "image/internal/imageutil" in jpeg/reader.go file,
and this code.

https://github.com/golang/go/blob/master/src/cmd/go/pkg.go#L867-L869

        for _, dep := range p1.deps {
            deps[dep.ImportPath] = dep
        }

deps["image/internal/imageutil"] is updated to data without error by import _ "image/jpeg".