Picture Frame Rectangle Tool

The following settings can be adjusted from the context toolbar:

Replace Picture—tap to display a pop-up panel from which you can select a file or Photos library item to insert into the picture frame.
Scale—tap and choose a method by which the frame's content is automatically scaled according to the frame's dimensions, or to turn off automatic scaling.
Anchor—tap to reveal a pop-up display, then tap on the square's corner, edge midpoint or centre to align the frame's content to that anchor point.
Clear Fill—when selected, the background fill is removed upon populating the frame (useful when framing transparent PNGs).
Size to Content—If the frame's dimensions are different to those of its framed image, tap this option to resize the frame to make it fit its content.

Some of a selected rectangular picture frame's attributes are editable via a related tool's context toolbar, a panel, or a menu command:

To set fill and stroke colours, use the Colour panel.
To set line style, use the Stroke panel.
To set Corner Type, use any shape tool's context toolbar.
To set transformation and selection behaviours, use the Move Tool's context toolbar. (Relevant settings: Enable Transform Origin, Hide Selection, Alignment Handles and Transform Objects Separately.)
To make the picture frame's shape editable, select Convert to Curves on the Edit menu.