I use phpthumb in a basic cms I set up for my client but every now and again it add a 1 pixel horizontal black line to the bottom of the image? I've tried re-uploading it but still the same weird problem.
Probably a rounding error in the resizing math. The example code on the imagecopyresampled manual page [php.net] even has the error... Basically, when you create the image it always uses an integer width and height, but when you resize one image buffer into another, you can actually provide float/decimal width and heights.
Just use ceil() when appropriate... you'll end up "losing" a row or column of pixels here or there, but that's better than the mysterious black line. :)