Create transparency on the edges of your pictures so they look like they have rounded corners.
Examples on getting started with regular and rounded rectangles.
Use the selection tool to paint rectangles with rounded edges.
Make a square shape with patterned edges.
Create a square, turn it into a path, then move it's nodes around.
Tip: Hold the Ctrl Key to create a perfect square, circle, or other uniform object.
An introduction to the Inkscape interface and how to use a few tools.
This tutorial uses Selection and Stroke to create interesting designs.