Photoshop is the best bet as far as I'm concerned.
It has paints, effects, styles, actions and filters to work with for creating all sorts of stuff.
You work in layers and save as a psd file so you can re-edit things if you need.
then you save out you finished work as jpeg or bmp, png or even a tiff.

I save out as jpeg.

You need to download her seamguides to work with and you find them in freestuff by doing a search for "seamguides" or by artist "Snowsultan".

Hope this helps. :)

Let me introduce you to my multiple personalities. :)
     BluEcho...Faery_Light...Faery_Souls.