The current coal mine graphic has some white patches on the non-snow image in one rotation that come from not having replaced the white with the background colour in the post-rendering stage. Attached is a fix:
(https://raw.github.com/jamespetts/simutrans-pak128.britain/f1d3b0f6324dac01d23b9b02bcd8c7898bbc818c/industry/victorian-coal-mine.png)