For those who want to convert a few SMILES strings to images, you can also use the CDK 1.5-based Depict utility from John May (www.simolecule.com/cdkdepict/, GitHub). It provides various options and outputs Scalable Vector Graphics (which can be easily converted into other formats).
For example, caffeine with title: https://cdkdepict-openchem.rhcloud.com/depict/bow/svg?smi=CN1C=NC2=C1C(=O)N(C(=O)N2C)C%20caffeine&abbr=on&suppressh=true&showtitle=true&zoom=1.3&annotate=nonehttps://www.simolecule.com/cdkdepict/depict/bow/svg?smi=CN1C%3DNC2%3DC1C(%3DO)N(C(%3DO)N2C)C%20caffeine&abbr=on&hdisp=bridgehead&showtitle=true&zoom=1.6&annotate=none
Thus, with the basic web API you can create a script to convert all SMILES strings too, e.g. using the RCurl package. This StackOverflow post explains how you convert the SVG to other formats.
However, since you probably prefer a pure R-based solution, please do have a look at the rcdk package.