Sounds like you are using an older version of WSI.
WSI 1.3.6 has this ability in the GUI.
In your case either upgrade or go to
inetpub/ftproot/rapport/ and delete the no longer needed images from the appropriate subfolder.
For example a V90L image would reside in inetpub/ftproot/rapport/Wyse9V92. Do not delete the whole folder if it contains more than one image. If you delete the complete folder all V90L images are gone, but the folder will be newly created when adding a new V90L image.