As Eugene commented, I had to find the target frame and do more work. There is no method to add images to a frame, only to a slide. I went into the methods and succeeded as follow:
DrawPageElement drawPageElement = slide.getOdfElement();
DrawFrameElement drawFrame = OdfElement.findFirstChildNode(DrawFrameElement.class, drawPageElement);
DrawImageElement image = drawFrame.newDrawImageElement();
OdfPackage mOdfPackage = odp.getPackage();
String imageRef = "/some/path/to/chart.png";
String packagePath = odp.getDocumentPath() + OdfPackage.OdfFile.IMAGE_DIRECTORY.getPath() + "/" + someMethodToCreateRandomString();
mOdfPackage.insert(new URI(imageRef), packagePath, OdfFileEntry.getMediaTypeString(imageRef));
packagePath = packagePath.replaceFirst(odp.getDocumentPath(), "");
URI uri = new URI(AnyURI.encodePath(packagePath).toString());
image.setXlinkHrefAttribute(AnyURI.decodePath(uri.toString()));
image.setXlinkActuateAttribute("onLoad");
image.setXlinkShowAttribute("embed");
image.setXlinkTypeAttribute("simple");
I was hoping for something closer to the GUI because I think I have missed some styles and better way to find frames. But anyway it is not bad.