what version of messenger is this? You might want to take a look at how I replaced the Shopping trolly icon in mobile chat windows in Vistafy, if you download and install it open 920 Definition and Style .txt's and search (Ctrl+F) buycreditsbtn and see in the style file the ButtonIcon part for it, I had to rename the icon resource, you'll see what it was renamed too out of frustration mostly
![:P](images/smilies/msn_tongue.gif)
(some dodgyness of WLM) and reference it in the definition.
This way you can circumvent the RLE restrictions in 9.0 and 8.5 if necessary