Inserting content into template placeholders
While a template placeholder is selected - such as a photo/picture placeholder - and you want to insert a photo from your library straight into that placeholder on the page, you should be able to select the photo within your library and press 'insert', thereby pasting directly into that box. This should also apply to text box placeholders and text content available within your library.
CATONNE Sandrine commented
The placeholder option would definitely boost the slide production productivity if an Image or Pictos libraries are managed with Slide Proof. Most of our users at Wavestone are waiting for this feature !