Change opacity for photoshop generated png

[mortgage]

I'm trying to compose two images after changing opacity for one of them (created by Photoshop CS 5.5). But result is strange.
My file has transparent background. I saved it as 'png-24'.
Note appeared white background.
Please, help — what am I doing wrong?

Code: Select all

convert logo: \( water3.png -alpha on -channel A -evaluate set 75% +channel \) -composite results.jpg

[snibgo]

Water3.png has opaque blue, transparent blue and transparent white. You set all the pixels to 75% opacity. So this gives 75% opaque blue and white. Perhaps you would prefer to multiply opacity by 0.75.

[mortgage]

I guess '-evaluate set 75%' is applying only to alpha channel because of '-channel A'. Why not?
How can I 'multiply opacity by 0.75'? Something like this?

Code: Select all

-channel A -evaluate multiply 0.75 +channel

Is it equivalent for 'normal' blend mode of Photoshop? I need such mode.