When you resize you do lose clarity, most software has different ways of doing it so low / medium / high quality settings (you may want to take a look through the software helpfile).
 
Also when you save to a new file you may add blur due to the compression of the data (the way the data is made smaller for the final file). Certain formats such as jpg use a compression that will blur the image more every time you save it and it has quality settings too (a trade of quality versus size), so while working it's best to use an image type that is lossless such as PSD(photoshop), PNG or XCF (there are loads of others) and only convert once to jpg

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Toolset: Blender, GIMP, Indigo Render, LuxRender, TopMod, Knotplot, Ivy Gen, Plant Studio.