jQuery provides a number of functions for animation and effects, which are listed in its documentation.

Changing visibility

For simple visibility changes, you can use hide() and show():
$("#pic").hide();
$("#pic").show(); (See example)
You can also use toggle(), which will decide whether to show or hide based on the current state: $("#pic").toggle(); (See example)
You can pass a duration into any of those functions, and jQuery will then animate the changing of the visibility: $("#pic").toggle(1000);
You can also use slideDown(), slideUp() and slideToggle() for sliding effects (See example) or fadeIn(), fadeOut() and fadeToggle() for fading effects (See example).
You can pass a callback function as the second parameter to any of those functions, and jQuery will call that callback function when the animation is complete:
$("#pic").toggle(1000, function() {
    $("body").append("It's here!");
});
You can also chain multiple effects together and call delay() if you'd like a delay between them:
$("#pic").slideUp(300).delay().fadeIn();

Custom animation

If you'd like to animate specific CSS properties, you can use animate():
    $("#pic").animate({
       width: "70%",
       opacity: 0.7,
       padding: 20
    }, 1000);
Note that you can only animate CSS properties that take numeric values - so you can't use this to animate properties like 'color'. (See example)
You can also attach various callback functions in animate(), if you'd like to find out when the animation has progressed or is complete. Check the documentation for more details.

Animating responsibly

Animations should always improve the user experience, not make it worse. Animations should help users understand something about the state of your web app or add a touch of fun - they should not slow down the experience unnecessarily and frustrate the user. You can ask users for feedback about your use of animations or work with a designer to decide how and where to animate.
If you know your user is on a device that doesn't perform well with animations, you can set $.fx.off to true.