You need to upload a new List.xml file as well as the PNGs (a full image + thumbernail image) and put all under the relevant folder.
The folder depends on the IP phone model.
For example, the folder for :
- 7906/11 is Desktops/95x34x1 (7906/11 IP phone customization)
- 7941/42/61/62 is Desktops/320x196x4 (7941/42/61/62 IP phone customization)
- 7945/65 is Desktops/320x212x16 (7945/45 IP phone customization)
- 7970/71/75 is Desktops/320x212x12 (7970/71/75 IP phone customization)
- 8961/9951/9971 is Desktops/640x480x24 (8961/9951/9971 IP phone customization)
- 88xx is Desktops/800x480x24 (8800 IP phone customization with attention to bug CSCus27001)