Try adding the truecolor parameter when you create your images: 

$im = new GD::Image(1000,100, 1); ## Note the extra parameter!

[download]

From the GD docs:

The optional third argument, $truecolor, tells new() to create a truecolor GD::Image object. Truecolor images have 24 bits of color data (eight bits each in the red, green and blue channels respectively), allowing for precise photograph-quality color usage. If not specified, the image will use an 8-bit palette for compatibility with older versions of libgd.

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In reply to Re^3: How to improve image resolution by BrowserUk
in thread How to improve image resolution...Continued by chakkaln

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