choose correct thumbnails size for mediawiki img #527

Job	Run time
build-f3discovery-doc	1m 52s
build-microbit-doc	1m 54s
build-book	26s
build-microbit-chapter (05-led-roulette)	34s
build-microbit-chapter (08-i2c)	39s
build-microbit-chapter (10-punch-o-meter)	42s
build-microbit-chapter (07-uart)	42s
build-f3discovery-chapter (08-leds-again)	28s
build-f3discovery-chapter (07-registers)	28s
build-f3discovery-chapter (14-i2c)	21s
build-f3discovery-chapter (09-clocks-and-timers)	27s
build-f3discovery-chapter (05-led-roulette)	28s
build-f3discovery-chapter (06-hello-world)	27s
build-microbit-chapter (09-led-compass)	35s
build-f3discovery-chapter (11-usart)	33s
build-f3discovery-chapter (16-punch-o-meter)	27s
build-f3discovery-chapter (15-led-compass)	25s
	11m 28s

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