Users images resize? slider...

When my users post a image in a thread thats big, the thread will only let us see half of picture....how do I add a slider so I can slide the image so i can see other half?

Code:

img:hover {
background: 0px -100px;
transition: 2s;}

and

Code:

img {
transition: 2s;
}

if these codes are in the css alrzady just add the things between the {}

Our forums have the same issue. I think we use the most basic CSS code that came with the forum when it was created. Where in the CSS file do I put the code above? Which section?

i see now that these codes won't work

but you can add this anywhere

Code:

img {
max-height: 300px;
max-width: 300px;
-webkit-transition: 2s;
-moz-transition: 2s;
-o-transition: 2s;
transition: 2s;
}
img:hover {
max-height: 750px;
max-width: 750px;
-webkit-transition: 2s;
-moz-transition: 2s;
-o-transition: 2s;
transition: 2s;
}

this will make images bigger when hovered