Why is 相 pronounced with a fourth tone in some cases? I notice that 相 is pronounced xiang1 in most cases, but xiang4 in a few, mostly less-common words and phrases. The most commonly-used examples seem to be 照相 zhàoxiàng 'photo'; 照相机 zhàoxiàngjī 'camera'; 相片儿 xiàngpiān(r) 'photo'; 相声(儿) xiàngsheng(r) 'comic dialogue' (see this question); and 相 xiàng 'elephant' (the piece in Chinese chess).

Usually (sometimes?) in Chinese a change in tone tracks some differentiation in meaning - is there any such here that might help me to remember which pronunciation is which?
相

[Xiang1]
surname Xiang

[xiang1]
each other,
one another,
mutually

[xiang4]
appearance,
portrait,
picture

You should look into getting a Chinese reader. http://www.loqu8.com has one for free that is really good. It uses the CC-CEDICT dictionary which is really good. If you would like more concrete examples, they're all going to be in Chinese, but there they are nontheless. http://stroke-order.learningweb.moe.edu.tw/advExplain1.do?big5=ACDB

Enjoy.^^