From the Navigator

On the Navigator ribbon, click Content tab, and then locate images. Select one and then drag it over an existing image in the article.

From the Shell main tab over Clipboard

On the Shell main tab, click the clipboard icon in the listing on one image. Return to Articles main tab, and then drag from the Clipboard tab over an existing image in the article.

From the Shell main tab directly

On the Shell main tab, select one image, and then drag over the label of the Articles main tab, hold while it switches to the Articles main tab, and then drop it over an existing image in the article.