I tried to play with the .dds file for you, but that particular image (\images\icons\icon_transp1.dds) is used as the background for not just bags and boxes, but also all of your equipped items as well as the icon in the examine window and some buffs. you might be able to point it toward something different.
this line in particular may be of interest:
"\eq2ui_IconStyles.xml"(13):
Code:
<IconStyle BackGlowRect="210,0,252,42" BackGlowResource="/images/icons/icon_transp1.dds" BackRect="84,42,126,84" BackResource="/images/icons/icon_transp1.dds" description="Same as button but no iconrect" FrameRect="126,42,168,84" FrameResource="/images/icons/icon_transp1.dds" MouseOverRect="84,84,126,126" MouseOverResource="/images/icons/icon_transp1.dds" Name="inventory" PressedRect="168,42,210,84" PressedResource="/images/icons/icon_transp1.dds" ProgressRect="84,42,126,84" ProgressResource="/images/icons/icon_transp1.dds"/>