Its best to stick to default size images when first learning, you can use the 'New skin" original images as size templates.
You can also experiment with image size as long as the states line up you can try going bigger or smaller, some images in WB need to be within certain dimensions, some don't. You can pretty much make them any shape, some shapes will respond well, some wont.
A LOT of skinning is trial and error, something every skinner has to get used to and be at peace with.
The predominant adjustments you will make in Skinstudio are sizing and Text margins, these are found on the tabs for each section\image
There is a little arrow to the right of the Edit image button that you need to click to open up a menu where you can choose to import an image tahat you have made in Photoshop and saved to your desktop.
As has been said, the best way to learn is to download and look at others skins and how they are assembled in SKS.
Also make sure you have associated Photoshop as your default editor in Skinstudio preferences dialog.
