The resource manager is the window where you import the graphics. You don't need a tutorial, it's quite simple.
When you import your graphic a window pops up displaying it, you have to LEFT CLICK on the background of the image to make it transparent, this tell RMXP that you want it transparent.
So remember, the Resource Manager is where you import the graphics/audio.
There you go!
Now you can import stuff like cute cat girls in mini skirts with their tails hanging between their legs as they walk and their hips swinging side to side with it, or a battle of the character with a short skirt and a sexy pose in battle...Of course, RMXP isn't great for those kinds of games...