That's strange, I'll check it.
As for the next major release, for now I'm interested, what building states/flags you'd like to be able to provide custom images for.
For example, current list of what can be overridden is below (some buildings are just missing currently). Images for buildings or variable sizes (bridges, etc.) will be properly stretched and must contain parts for corners, edges and middle area.
archerytarget, archerytarget_ns, archerytarget_sn, archerytarget_ew, archerytarget_we, armorstand, axle_horizontal_ew, axle_horizontal_ew-active, axle_horizontal_ns, axle_horizontal_ns-active, axle_vertical, axle_vertical-active, bars_floor, bars_vertical, bed, bed-dorm, box, bridge, cabinet, cage, chain, chair, coffin, door, door-locked, floodgate, floor, gear_assembly, gear_assembly-active, grate_floor, grate_wall, hatch, hive, nestbox, rollers_ew, rollers_ew-active, rollers_ns, rollers_ns-active, rollers_sn, rollers_sn-active, rollers_we, rollers_we-active, screwpump_ew, screwpump_ew-active, screwpump_ns, screwpump_ns, screwpump_ns-active, screwpump_ns-active, screwpump_we, screwpump_we-active, slab, statue, support, table, traction_bench, wagon, wall, weaponrack, well, windmill, windmill-active, window_gem, window_glass
+workshops/furnaces and custom ones