Inserting Multiple Pictures is Broken - only one picture treated as picture
If you insert multiple pictures all at once into Excel, only the last inserted picture is treated as a picture (e.g. the Picture Tools tab is activated). All the other pictures are treated as objects, and some options are not available (such as the Picture tools tab and the contextual menu item "Size and Properties").
Furthermore "picture pictures" can only be grouped with other "picture pictures", and "picture objects" can only be grouped with other "picture objects". Neither can be grouped with drawing objects.
See attachment "PictureObject.PNG" for a picture that was treated as an object, and attachment "PicturePicture.PNG" for a picture that was treated as a picture.