Also, what fomat & size is the photo?
If you can, optimize in PhotoShop or another photo editor. If it's a huge, hig-resolution file, you might be feeding FCP more information than it needs, making the problem worse than it needs to be.
I try to keep photos no larger than 1440x960 (that's double 720x480 resolution) when working in SD. I'm assuming this is an SD project. HD doesn't have the same issues with "too much detali" as SD. Anyway, double the pixels leaves room to do moves on the photo if that's what you're looking for.
If you just need it to fill the frame and not move, don't make it any larger than it needs to be. Let FCP use it's resources judiciously. Only feed it what it needs. Otherwise it's doing a bunch of math in the background that it doens't really need to do, and that can slow down your processing.
I've also been using the .png file format lately, FWIW.
If you're still having issues, you can go back to your photo editor and apply the gaussian blur to just the parts of the photo that are giving you grief. This leaves the rest of the photo crisp.
deb