First make sure all images are in RGB Color Mode - not Grayscale, Indexed or CMYK. Note the 'color mode' isn't the same thing as 'color profile.'
FCP's resolution limit is 4000 pixels so you definitely want to keep images smaller than that. Since your Sequence settings are for 1080, it would be best to limit the image's sizes to just what you need for those settings. i.e.; if you don't plan to zoom in on any of the images, then too many pixels more than 1920x1080 would be a waste. If you plan to zoom in or pan across, then you may want to double the resolution of your Sequence.
As for containing all of the information on the photo, be aware that still images seldom have the same aspect ratio or orientation as video. So if you have a vertically oriented photo and want to show it all from top to bottom, there will be black on each side once in an FCP Sequence.
-DH