GOCR is very easy to use and it's callable from the command line. Just type gocr -h and you will have all the available commands with the needed information on how to use them.
Tesseract's image processing is very rudimentary, in order to get the most out of it you need to use a preprocessor or use an image that's already been processed.
A for humans perfectly readable image 100 dpi results in a huge number of failed characters even if source is free from physical scan artifacts (i.e. print to file). If possible, it's better to use 300 dpi image files.