You cannot judge the quality of any image in the Canvas on the computer.
You must use an external video monitor and RENDER all effects and stills to view the full quality.
Any graphics imported are much better quality if they are not too small or too LARGE in relation to the size they actually used.
An imported TIFF (or PiCT, etc.) that has too MUCH resolution will look “buzzy” or “dirty” because of the relatively low resolution of NTSC/PAL video.
You should make the size of any image as close to the “final” size you will use in FCP.
Even if you NEED a larger image size (you plan to “zoom in” at some point) you can apply a small amount (between 0.25 to 2.5 percent) of Gaussian Blur to any graphic that is too “crisp” to reduce the visual “buzz”.
But again, you must view on the external monitor (should be used at all times during the edit to judge quality) and rendered to make sure all is well.