How to create Static block that shows an random image

Create a template file.
Let's call it random.phtml.
Put it in app/design/frontend/base/default/template/random/random.phtml.

The contents should look something like this:

<?php 
$images = array('img1.png', 'img2.png', ....);
shuffle($images);
$image = $images[0];
$url = Mage::getBaseUrl('media').'path/to/folder/'.$image;  
?>
<img src="<?php echo $url?>" alt="" />

make sure you put all your images in media/path/to/folder. (feel free to change the name of the folder).

Now you can use this in 3 ways.

Either inside a static block:

{{block type="core/template" template="random/random.phtml"}}

either in any other template:

$block = Mage::app()->getLayout()->createBlock('core/template')->setTemplate('random/random.phtml');
$html = $block->toHtml();
echo $html;

either in a layout file:

<reference name="left"><!-- not necessarily left. can be right, content, header... -->
    <block type="core/template" template="random/random.phtml" name="random-image" as="random-image" />
</reference>

If you want to create static block in your form steps:

1) go to -> Namespace_Module_Block_Adminhtml_Module_Edit_Tab_Form extends Mage_Adminhtml_Block_Widget_Form

Add code : $fieldset->addField('filenamelogo', 'text', array( 'name' => 'filenamelogo', ));

  $form->getElement('filenamelogo')->setRenderer(Mage::app()->getLayout()->createBlock('module/adminhtml_module_edit_renderer_logo')); 

2) Create a file with name Logo.php on Block/Adminhtml/Module/Edit/Renderer/Logo

3) Add code in Logo.php class Namespace_Module_Block_Adminhtml_Merchant_Edit_Renderer_Logo extends Mage_Adminhtml_Block_Widget implements Varien_Data_Form_Element_Renderer_Interface {

public function render(Varien_Data_Form_Element_Abstract $element) 
{ $html .= ' your html or php code '
return $html;
}}

You need to create multiple static blocks, with a mutual identifier prefix:

Like:

BannerIdentifier_1

BannerIdentifier_2

BannerIdentifier_3

BannerIdentifier_4

...

BannerIdentifier_N

Then pass your image to whatever block:

<a href="http://mystore.com/my_new_product/"><img src="{{media url="wysiwyg/Banner/ban_1.PNG"}}" alt="Some alt text"  /></a>

Then call that static block on phtml file:

<?php echo $this->getLayout()->createBlock('cms/block')->setBlockId('BannerIdentifier_'.mt_rand(1, N))->toHtml() ?>

I am not sure if this is best, but it can be a way of doing this.