gocr is an application that allows you to do OCR on graphical files under Linux. However be warned that it may not work for graphical files other than .pnm files. To convert .png files to .pnm files, use the command convert from the ImageMagick package.
Example: In the following example, a screenshot from kwrite is recognized. The only sentence written in kwrite is "The quick brown fox jumps over the lazy dog.". By using e.g. scans instead of screenshots, you will get worse results, as scans are already an A/D conversion.
$ gocr ocrtest.pnm E_Ie €d_t __ew _ookmarks _ools _ett_ngs Help ____ _U __:r_,;_ _ ____ _ e__ h_ The qu_ch brown fox _umps over the lazy dog.l