29.97fps timeline / keyframed stills 23.98fps ... why?

I'm inserting key-framed animated stills in a standard 29.97 fps timeline. I'm getting some shimmering on the stills with slightly higher contrast edges. I know this might be the size of the stills (I'm trying to keep them fairly small (HD) in resolution, or the specific animated moves ... but,

I'm just wondering. When I look at the properties of the stills, they are all 23.98 fps. Everything else on the timeline and the timeline itself is 29.97 fps. Why would the stills be 'supposedly' a different framerate? And is this something I can change, or should bother changing?

All ears,

Ben

It would help to reduce the contrast and thin lines where possible. Apply a tiny bit (.05) of gauusian blur and maybe use cc to reduce the high contrast.