Go to the Back End admin - Auctions & Items > View Auctions
Go to the items section of the correct auction
Search for the item you wish to delete the image for and tick the pertinent box for editing
Note: You can only edit an item that is in DRAFT status - If an Item has already been set to LIVE / PREVIEW status you will need to select (tick) the relevant item and click Draft > Confirm
Then click on Bulk Edit as below
Once you are in the Bulk Edit screen, scroll to the right and click the 'picture' link found in the Attachments column (see below)
In the pop-up box that appears, click on the Images tab
You can then choose to delete individual images by clicking on the 'x' button (top right of image) and then Save
Or
Highlight the images you want to delete, click Delete All Images and then Save
Click Save and Finish
The item will now be in PREVIEW state - you will need Approve the item to return to LIVE status