Open up Inkscape. Create a new layer called "Coastlines", and start drawing the outlines of your islands and continents.
If you have trouble coming up with good coastlines - or other terrain features, for that matter - I recommend looking at images from Google Maps. There are plenty of interesting terrain features in this world which can inspire you.
Now draw the continental shelf around your continents and islands on a new layer called "Continental Shelves" - the line where the deep ocean becomes shallower.
Next, create a layer called "Mountains" and draw the ridges of your mountain ranges - the lines where the highest points connect with each other.
Finally, create a layer called "Rivers" and draw some rivers which flow from the mountains to the coastlines. Select them all with Ctrl+A and give them a nice, blue color with Objects->Fill and Stroke: Stroke Paint.
We will add some further details to the map - different vegetation, cities, and so on - but that will do for now. Now export the layers as bitmaps:
Make the "Mountains" and "Rivers" layers invisible and export the rest as "OceansLayer.png".
Make only the "Mountains" layer visible and export it as "MountainsLayer.png".
Make only the "Rivers" layer visible and export it as "RiversLayer.png".