Hi guys,
many thanks but today I've find solution
The problem was the procedure that I’ve used to create bitmap pictures with Gimp; I don’t know if the new procedure it’s the correct way, but solve my problem.
For every newbe (like me

) some topics:
1) Yes, I use layers (inside .xcf) with transparency background, but there is one layer (filled with color ECE9D8) that I turn on before save bmp picture.
2) When I save .bmp file I use "Unisci livelli" (Eng. "Merge Layers" - I think)
3) MOST important I use "Opzioni avanzate" (Eng. "Advanced setting" - I think) and I select 24bits (R8G8B8)
I've included image about saving procedure (Gipm Italian Language) to better explain.
In previous procedure a) I don't have used a layer with color and b) I have used 32bits (A8R8G8B8).
I think the problem was an "incorrect" loading or recognizing of the bitmap inside the exe.
And now the question: this have solved my problem, but is it the correct way or is there another way?
A little note: if I don't use the layer filled with one color, the pictures on the toolbar have a white background.