What are the pixel dimensions of your images? If the images are smaller than your sequence dimesions and you’re resizing them, you might see some loss in quality.
Another thing to check is the RT (real-time) settings in FCP. There’s a little drop-down button on the top left corner of the sequence which, if set to “low” or “medium” will influence how your footage is viewed. This will be especially the case if you are looking at the sequence on an NTSC monitor. The “low” and “medium” settings only play one field. (This is actually really useful if you are trying to preview lots of layers in real time.)