What AirWave version are you running?
DE20546 sounds pretty close to what you ran into. This is fixed for 8.0.7 (when 8.0.7 is released).
The steps:
1) upload new logo into Device Setup -> Firmware & Files (there's an add new captive portal button at bottom of page)
2) set group to use new logo: from group -> select group -> basic -> aruba instant setting box -> captive portal logo
3) desired result is the captive portal logo gets uploaded. this can be confirmed also in the igc logs or vc/ap logs in /var/log.