Find all regular files under and below /somepath that have extensions PDF, TIF, TIFF, PNG, JPG, JPEG, BMP, PCX, or DCX, ignoring the case and excluding "*_ocr.pdf" files
find /somepath -type f -iregex ".*\.(pdf\|tif\|tiff\|png\|jpg\|jpeg\|bmp\|pcx\|dcx)" ! -name "*_ocr.pdf" -print0