Telltale watermarks allow to infer how a watermarked signal has been altered on the channel by analyzing the distortion applied to the watermark. We propose a new kind of telltale watermark for digital images that tracks the number of JPEG (re-)compressions. Our watermarks leverage peculiarities in the convergence behavior of JPEG images. We show that it is possible to generate or find combinations of pixel values which form so called "counter blocks". Counter blocks cycle predictably through a fixed number of states in subsequent JPEG compression and decompression operations. By combining counter blocks with different cycle lengths in one image, we are able to track the number of JPEG (re-)compressions in extended ranges. We evaluate the accuracy of counter blocks, discuss pitfalls when embedding them, and study the construction of counter blocks with specified cycle lengths.