If I delete a record from that table, how can I have dataassist/Digital File Pro automatically delete the associated files in the record from the images/bike-images/ directory? 
Here's what I'd do. Create a recordset that references the record that it to be deleted. That recordset should contain all the columns that contain the image names that you want deleted, let's say: img1, img2, img3, etc.
Then you'd need to apply the DFP Delete File Server behavior once for each file you want to delete. IN the Delete file user interface, enter the folder path relative to the page you're doing the delete on, let's say it one level up:
../images/bike-images/
and then in the File name field, click the dynamic button (lightning bolt) and then choose img1 from the recordset created above.
Rinse, lather, repeat for each image to be deleted.
DFP Delete file should appear in the code after the DataAssist Delete Record server behavior, and so, one thing to note is that if you are performing the record delete on the same page as you want the images to be deleted you may need to be aware of the order that the code is applied to the page. You won't want to have the Delete record have a redirect page specified as the redirect will occur before the files are deleted.
In DFP, there is a Server Redirect server behavior that you can set a redirect page in this situation. This redirect will be applied after the delete file server behavior.
HTH
