Minimum time increment in timeline?

I am working on a project that uses "flip-book" animation. That is, a series of sequential images plays on the timeline to simulate an animation. On dozens of slides, i could position the images 1/8 second apart. on one slide. The minimum increment I can place the images is 1/4 second.

If I use "timing" I can set the start time in 0.25 second increments, just can't drag. Is there a way to set the 'snap to" in the timeline?

Thanks for an help.