Actually you can it seems, just tried it. I just went in and edited the extension of that file to PNG and it just repacked itself.You have to open the Jar with Winrar.
(If your not familiar with winrar, you can not edit files that are compressed, so you need to extract it somewhere, convert it (its easy to just open it in paint and use save as to change file type) and then add in back into the Jar.)
My version of the game is the latest (with Redhead Mod) already has it listed as a .png so it might be faster just to get the mod if its fixed there or see if there is an update.
Pretty much latest version of winRAR iirc.