Image resizing "on the fly"

Sorry about the wording - I'm new to developing and don't know exactly how to phrase it so please bear with me...

I'm inserting an image gallery with HTML & CSS only. I want there to be thumbnails that when clicked "push" the thumbnails down and bring up a larger image of the thumb that was clicked.

*I've attached the HTML and CSS*Attachment 15395

I don't think I need script to do something like this, but I also think the code I have is quite a bit off from my desired effect.

Can anyone help??

Thank you!

(Also, sorry if this is in the wrong spot - it's my first post)