For anyone still interested in this topic... I just tested out imacros for chrome:
http://www.filetransit.com/view.php?id=114719 on OSX. recorded the following macro - then set it to loop. Depending on your connection speed, it should take about 10 mins per 100 files. This will basically delete the files starting from your lowest downloads.
VERSION BUILD=1100208 RECORDER=CR
URL GOTO=http://www.dreamstime.com/onlinefiles.php?pg=1&srchby=&orderc=downloads&orderm=ASC
TAG POS=1 TYPE=IMG ATTR=SRC:http://www.dreamstime.com/localization-images/en/btn_edit2.jpg
TAG POS=1 TYPE=IMG ATTR=SRC:http://thumbs.dreamstime.com/img/btn_disable.jpg
TAG POS=1 TYPE=INPUT:RADIO FORM=NAME:frm_disable ATTR=ID:rd_confirm
TAG POS=1 TYPE=TEXTAREA FORM=NAME:frm_disable ATTR=NAME:M_reason CONTENT=Going<SP>exclusive<SP>to<SP>istock
TAG POS=1 TYPE=INPUT:IMAGE FORM=NAME:frm_disable ATTR=SRC:http://thumbs.dreamstime.com/localization-images/en/btn_sbmit.jpg
Hope this helps someone!