Fix fallback image ignoring PREVIEW_PICTURES_DIR
Fix fallback image ignoring PREVIEW_PICTURES_DIR

Related to #20

codedust/gluon-firmware-wizard,freifunk-darmstadt/gluon-firmware-wizard,freifunk-darmstadt/gluon-firmware-wizard,freifunk-darmstadt/gluon-firmware-wizard,codedust/gluon-firmware-wizard,codedust/gluon-firmware-wizard