Ok this part is a little confusing. I asume you can export and you know it creates the folder "xetdata" which the dds. file inside.
The File name MUST stay the same, also if you export a image then login game then after that try to import the image it won't work, once you login the game the file names will change. So when you export the image you must edit it then import it before logging in.
To import a file do the following.
It will bring a brower window up, just select the xetdata with the edited dds. files (It must be inside the "xetdata" folder or it wont work)
It will take a second or 2 depending on how many files you need to import and once it's done it will display a chinese message, just click Yes and this will create a new SGL file inside xetdata, this SGL file will be the edited one so just replace it with the one in the client.
Always best to keep backups just incase you mess it up or something.
Remember you need to save the dds file as a dds file and in this format.
If you do not have the dds plugin for photoshop you can download it from NVIDIA
http://developer.nvidia.com/sites/default/files/akamai/tools/files/DDS_Utilities_8.31.1127.1645.exe
Remember DXT3 ARGB 8 bbp Explicit Alpha