This is from the main site:
http://www.lineage2.com/Knowledge/clan_actions.html
Edit Emblem
The Edit Emblem button allows a clan member with privileges to change the clan crest and clan insignia for clans that are level 3 and above.
The crest must be 16x12 pixels and saved in 256 color *.bmp file format.
So, basically, you have to have the .bmp file saved on your computer. Just type in the file location into the text bar (i.e., C:\stuff\l2\crest.bmp ).