Below i will try to explain how you can edit the alpha layers that came with your purchase to match your shape. In this example I'll use Photoshop but the same method probably applies to other graphic software like Gimp as well.
Download the Upper Body and Lower Body templates of the SL Avatar and open them in a graphic editing program like Photoshop. In SL, open the alpha texture/s that came with your purchase and save them to your HDD as .PNG and also open this in Photoshop. Drag a copy of the alpha texture (white/transparent) and place it on top of the SL avatar template as a 2nd layer. The borders of the alpha texture need to match up with the borders of the avatar template and should 'snap' into the right place when moving around.
Once the alpha texture is correctly placed on top of the avatar template, you can start adding or removing transparency. If you need to reduce transparency (show more skin), you can do so by drawing more white on the alpha texture (2nd layer) at the areas you want to make visible. If you want to add transparency (hide more skin), you can do this by erasing parts of the white alpha texture.
Now its time to test your edited alpha texture. Hide the background layer (= avatar template) so only the white/transparent alpha texture is visible and save this image as .PNG. Upload the image to SL, create a alpha clothing layer and apply your alpha texture.
You will probably need to edit the alpha texture multiple times before you get the desired result so its best to either use temporary uploads or log in to the beta grid to avoid unnecessary upload costs.
Of course you can also create a new alpha layer from scratch. A well explained tutorial on how to do this, can be found here.