The methods I now use are – for small images to prevent them from being scaled up I save them on a transparent psd the size of the project, this also allows you to position it in photoshop.
For cropping a project I change 1 of the dimensions in project setting and then in pan and crop – match output aspect (this stops the video becoming blured by scaling it), I then move the video into position to fill the black bar.
You have to only change 1 of the dimensions – save in a lossless format (Lagarith) then change the other dimension of the saved file and repeat the steps.