Before starting the conversion, go to the “Options…” dialogue box (via the View menu) and make these two changes on the General tab.
1. Under the OCR frame, check the radio button Enable images extracting.
2. In the OCR engine drop-down menu select the Perform Image-Based Conversion (OCR) option.
Click on the OK button, click on the Select All button on the toolbar, and click on the PowerPoint button (under Convert to file type) on the toolbar.
This article refers to Able2Extract and Able2Extract Professional.