When I import a still to FCP and the still is not the same ratio as my project, it appears on the Timeline with black bars on the side. If I crop the still to 853px x 480px for 16:9 it will fill the frame, but I have too many stills and it is very time consuming. Is there a setting that will allow automatic filling of the still frame width-wise in FCP?
If they are all the same size, just import them as they are, then put them in a sequence as you have been. Double click one of them to open it in the Viewer, and then adjust it's size up until it fills the frame. Then copy this first clip and select them all. Then right click on one clip in the sequence and select "paste attributes" from there select the "basic motion" attribute. This will apply that enlargement to every clip.