Always re-size photos to your timeline frame size before importing. If they're too large, it won't help, and could hurt when you have to shrink them. Making things larger looses image quality quickly, but making things smaller can still cause image quality loss, just not as quickly.
Also, the format of the photos is important. So 2 pieces of information would help here.
1- What file type is it, RAW, JPEG, PNG, etc?
2- What is its actual fame size?
You can try a couple of things in the mean time.
1- Download "Preference Manager" from Digital Rebellion (dot com) and use it to safely and easily trash FCP X's preference files. This won't hurt anything, you won't lose anything.
2- Go into the Event folder that photo is in, be sure to move all Render files into the Trash. Also perhaps move the photo itself out of the Original Media folder. Go into the Project folder and trash all Render files.
This will cause FCP X to show the photo as Offline/Missing when FCP launches, but that's OK. Just right-click it and choose "Move To Trash". Re-import once it is re-sized for your timeline, and be sure to select "Optimize" in the Import window.