How to change dimensions of slider/make image wider than slider?

Hi!
For one of the questions in my study, I have an image that contains 5 pictures in a row, basically showing a change in emotion, for participants to "slide" to where they feel.
The slider has 9 grid line points set to "snap to grid line".
I need these grid lines to be positioned a certain way underneath the image - the "1" directly in the middle under the first picture, the "2" directly between the first and second picture, the "3" directly under the second picture, and so forth until the "9" is directly in the middle under the fifth picture.
Right now the slider lines up directly with where the image starts, so it starts on the left line of the first picture in the image, instead of directly in the middle of it.
This happens even if I change the image dimensions using the rich content editor!
Does anyone know how to modify the image and/or slider in this way?
Thanks for any help!