The best tool to undertake the task at hand is the simpliest one that handles it
This freeware program helps one into easily converting the contents of files (.bmp, .css, .csv, .ec, .gif, .htm, .html, .jar, .jpg, .jpe, .jpeg, .jif, .jfif, .js, .json, .pdf, .png, .svf, .svg, .tif, .tiff, .xls, .xml, or .xul) into the corresponding data uri (universal resource identifier), which, subsequently, can be used embedded in both html pages (.htm or .html) and cascading style sheets (.css).

In addition, this program can keep state, so that a batch job can be paused and, then, continued, at a later time.
Windows XP, Windows Vista, Windows 7, Windows 8, Windows 8.1, and Windows 10
Portable installation:
Uncompress the zip file to the folder from where you want to run the program (assuming you have both write and execute permissions on that folder) and run it from there.