For image objects with no defined border, it's impossible to make a percise selection with Paint.NET's "Ellipse Select" and "Rectangular Select" tools -- the shaped-based selection tools are too rigid to select the edges of a complex object. Paint.NET's "Lasso" tool works well for selecting objects with rough borders, but falls short in tracing objects with broken or muddled borders. But with the help of Paint.NET you can select objects based on their colors, so you'll have no problem selecting objects like clouds and liquids.
Start Paint.NET. Click on the "Open" button in Paint.NET's toolbar to load your image or Paint.NET project file into the program.
Select the "Magic Wand" tool from Paint.NET's "Tools" window -- the "Magic Wand" tool's configuration options will appear in Paint.NET's toolbar.
Click on the area you'd like to remove from your image. Use the "Tolerance" slider in Paint.NET's toolbar, to adjust the tool's sensitivity. Drag the slider to the left, if the "Magic Wand" selects too much of the image. If the tool selects too little from the image, drag it to the right to increase the tool's sensitivity. Make your selection again, after fine-tuning the tolerance.
Click on the "Edit" heading. Select the "Cut" option from the "Edit" menu to remove the selected portion of the image.
Click on the "File" heading, and then select the "Save" option to store your changes.