It's quite easy to display magento's sub-categories under top level category page. I keep loosing the previous work, so i am writing here so i can find it faster next time. And, You can use it too if you want to :) Any suggestions is welcome.
Add below code on file: catalog/navigation/sub_category_listing.phtml
So, far I know Magento e-Commerce has no support for image resize other than those for product images. But no need to fear as we can use same image resizing object and customise it to suit our need.
Just follow the following post and you will have image resizer in fraction of time you will spend on researching and writing your own as i had.
Do you want to move magento's sidebar mini cart to header? If you said "yes' then follow the following post. We are just using exisitng sidebar mini cart and modified to display on the top header. We have removed some of the Magento's code from the cart which we didn't need in our case, if you need it you can re-include from sidebar's phtml file that comes with magento e-commerce's base template.
app\design\frontend\your-package\your-template\layout\checkout.xml
<reference name="header">
<block type="checkout/cart_sidebar" name="cart_cartheader" template="checkout/cart/cartheader.phtml" before="-">
<action method="addItemRender"><type>simple</type><block>checkout/cart_item_renderer</block><template>checkout/cart/cartheader/default.phtml</template></action>
<action method="addItemRender"><type>grouped</type><block>checkout/cart_item_renderer_grouped</block><template>checkout/cart/cartheader/default.phtml</template></action>
<action method="addItemRender"><type>configurable</type><block>checkout/cart_item_renderer_configurable</block><template>checkout/cart/cartheader/default.phtml</template></action>
<block type="core/text_list" name="cart_cartheader.extra_actions" as="extra_actions" translate="label" module="checkout">
<label>Shopping Cart Cartheader Extra Actions</label>
</block>
</block>
</reference>