To add some prefix or suffix to generated images (for example, add _thumb after each filename) check Use Prefix / Suffix checkbox, select prefix or suffix mode from drop-list and type your text in Prefix / Suffix control. You can see a sample filename at the bottom of page.
Using this option you can number all the images starting from appropriate value. To do this, check the Use Autoincrement checkbox and select starting value (by default, zero). Select minimal number width if necessary — this will cause leading zero filling if number's width less than specified. Sample filename can be viewed at the bottom of page. Check Ignore Source Filename to exclude the source file name from the target name.