If you want to get rid of all the default images first, then you'll need to go into C:\Program Files\Common Files\Enterbrain\RGSS\Standard\Graphics and delete all the image files in the folders under Graphics.

If you want to keep the default ones there (some are quite nice, and it's always good to have some things to fall back on in case, say, you're unhappy with the new images), then obviously don't delete them.

Whichever of the two you decide to do, you'll need to do the following to import new images:

- Open up RMXP
- Open up your project
- On the toolbar, you'll see an icon that looks like a folder with three documents to its bottom right. Click it.
- This brings up the Resource Manager.
- Choose the folder into which you'll import your images (if it's a tileset, then it goes into Graphics/Tilesets, and so on).
- Click import, and browse to the file.
- Select it.
- Hit OK.
- Repeat as necessary.

That should work perfectly.