Every object has a mass and a contour. Drag a rectangle around the image. To make fine adjustments, select the image and your arrow keys to move it in the desired direction. Illustrator has a preview mode and an outline mode. Preview is the normal view showing the images with colours and textures etc.
Outline shows only the contours.
This is a major help in finding edges of shapes in order to select an object when the design has become complicated. But here I've put everything on the same layer and I need to make sure the picture is always on top. Now you can create the colour of mat you want, select mat, go to no stroke and select fill, this gets rid of the black line. Open swatch palette, and click a colour of your choice. Open the colour palette and adjust the mix.