Giorgio, I have used Corel Photo paint and Paintshop pro in the past.
I did like Paintshop, not sure about the latest version but at the time it had almost the same tools as photoshop, and for the price it was a real bargain. I just stuck with photoshop mainly because is one of my daily working tools, so I am very comfortable with it.

Learning to use any image editing software well will give you good generic skills that you can port to any specific application anyway.

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-'I curse the day my curiosity led me to investigate the strange stain inside
the atomic acelerator chamber'-
The Nuked Bug