A: 

What you are looking for is not transparency but alpha. Alpha is another color channel that goes along with the red, green and blue colors to determine the transparency level (just transparent is visible or not as opposed to being able to set the level of transparency). Check out imagecolorallocatealpha

Jonathan Kuhn  2010-10-14 16:06:19
This does not answer the question... At least, you gave me a hint that I should edit the title
BlaXpirit  2010-10-14 16:07:51
Sorry, I thought you were trying to add transparency to the text, in which case you would use the imagecolorallocatealpha when setting the $c00FF00 variable. The images you put in your request just give me a time out error and so I couldn't see the example images.
Jonathan Kuhn  2010-10-14 18:14:54
+2  A: 

You need to call imagesavealpha() against your imported PNG ($sim) before merging, as well as against the final image.

Stan Rogers  2010-10-14 16:26:51
I need to experiment with it, because now I got an opposite result: white background with transparent images :D
BlaXpirit  2010-10-14 16:41:48

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