Oh thank you for your answer Cowboy, but that's not exactly what I meant, maybe I should have used the word "borders" to be right...
I tried your thing Gilraen: I created a new layer, filled it with white, and then I took the "rectangular marquee tool", made a selection inside my graphic, cleared it and so I had a one-pixel frame around my graphic and I played with the blending!
My question has found its answer!
Thx again for both your help!