If your creating your own Sencha Touch 2 theme, it's better to use SASS. They documented this part very well and you are good to go in less then a hour.
If you done this, you need to add this in you SASS style file:
Now, you NEED to save you icon (60x60) in a specific location. Inside the root of Sencha Touch, the correct path is:
resources/themes/images/default/pictos
.x-tab .x-button-icon.newicon,.x-button .x-button-icon.x-icon-mask.newicon{-webkit-mask-image:url(../sencha-touch-2.0.0-commercial/resources/themes/images/default/pictos/compose2.png)};
Put this in ur senha page and in code make following changes:
I know the thread is old, but maybe more people will have this problem. I think the main problem is that you can't just edit your sass file, but you need to compile it as well.