customize images on frontpage

Good day folks,

We are using your plugin by suggestion from a friend who thought this would help us to accomplish what we are looking for.

We want to add css to specific images on the frontpage of the website. With the ID of the image we thought this would be feasible.

We tried the following code:

.wp-image-IDnumber {
code in css
}

#wp-image-IDnumber {
code in css
}

#img-IDnumber {
code in css
}

But none of these options work.

Maybe we are understanding this wrong or maybe we need to change the format of the code added, but definitely your help to find this out will be greatly appreciated.

Thank you for your help and interest.