I came across the blog post http://juretriglav.si/standards-for-graphic-presentation/ and I was surprised to find that there were actually standards for creating scientific graphics and they were created in 1914. 100 years ago! Wow!
Generally graphics are not thought as important as the textual components of the paper/book/blog post. This is a mistake. Prof. Till Tantau’s (creator of the excellent TikZ/PGF graphics system) suggestions for creating good graphics, under section Guidelines of Graphics in http://www.texample.net/media/pgf/builds/pgfmanualCVS2012-11-04.pdf are also definitely worth reading. Check out the TikZ/PGF gallery at http://www.texample.net/tikz/examples/.