I've never used GD, so I won't comment on your code, but I do know my image editing.
If your script isn't outputting any errors, and the final image does "slightly resemble the source file", it may be a problem in the way you're resizing, rather than any errors in the code you're using.

I'll explain:
If the original image is significantly larger than the final thumbnail, then you may need to resize it in two or three steps, to maintain image quality. I personally don't like to reduce an image by more than 25% in one pass, particularly if the final image is very small (and 150x150 is quite small).

I can't provide GD specific code for this, but what I suggest you could do is something like this,
(Please note, this is not real code, this is to explain the theory to you)

open image;
get image_width;
if (image_width > (150 * 4)) {
  change_image_size to "25%";
}
change_image_size to final_thumbnail_size;
print_image;
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