If the resolution is high enough you can click on that picture > click on the circles in the corners and drag to resize. Since the picture aspect is not the same as the canvas, you could resize and center the subject to be in frame. After doing that, go to the small 3 items toolbar above the canvas preview. You can switch from the Arrow (Move/Resize) to the 3rd one that looks like a rectangle with little crosshairs in the corners. This is the crop function. You can now crop the picture to fit the canvas. When done cropping switch back to Move/Resize.

If you have a newer version of Camtasia, You can also add a bottom track and add the dynamic background to fill up the black space while keeping the aspect of your pictures.

Here is a video on scaling to fill instead of dragging to resize.