Open an image in Single Photo View.
Click the edit icon to open the Edit Panel.
Twirl down the arrow next to Vignette.
Adjust the Amount slider to darken or brighten the edges.
Use the Size slider to set the size of the vignette.
Use the Roundness slider to change the shape of the vignette.
Learn More: https://manual.mylio.com/topic/adding-a-vignette