Broken thumbnails (sometimes) with TimThumb

99% of the issues I’ve had similar to this were when TimThumb simply ran out of memory. There’s usually a direct correlation between image size and memory usage. The bigger the image is, the higher the chances it will run out of memory.

To get a better idea of the error, access the timthumb url directly in your browser (not to the full-size image, to the generated one). Usually Timthumb will spit-out an error that will let you know why it failed to generate it.