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This paper gives a novel wavelet image two-line (Wi2l) coder that is designed to fulfill the memory constraints of a typical wireless sensor node. The algorithm operates line-wisely on picture data stored on the sensor's flash memory card while it requires approximatively 1.5 kByte RAM to compress a monochrome picture with the size of 256x256 Bytes. The achieved data compression rates are the same as with the set partitioning in hierarchical trees (Spiht) algorithm. The coder works recursively on two lines of a wavelet subband while intermediate data of these lines is stored to backward encode the wavelet trees. Thus it does not need any list but three small buffers with a fixed dimension. The compression performance is evaluated by a PC-implementation in C, while time measurements are conducted on a typical wireless sensor node using a modified version of the PC-code.