You can use a colour matte to make temporary placeholder images (e.g., to see how potential images synchronize effectively with musical beats, etc.) To set up the Colour Matte, go to File > New > Colour Matte. Similarly, you can click on the New Item Button at the bottom-edge of the Project Window. When the Colour Picker opens, choose whatever colour you wish and give the matte a name. The matte will then appear in the Project Window. Premiere will give the matte the same duration as the Default Still Image Duration in the Preferences (go to Edit > Preferences). However, you can also change the length of the matte later in the Timeline.
Image size: If you wish, you can create a colour matte that is not full screen, which, when it’s in the Timeline, will appear as a square or rectangle within a larger frame.
To do so, when you create the matte and pick the colour, for Width and Height, enter values that are less than full-screen, such as 400 x 400.