Try running the Unused Files report (Tools > Reports > Unused Files) menu item. This is a reliable source of files that you can delete in Windows Explorer. If the images show up there, you can delete them.
As far as your other files that you can't delete, my guess is that there is still some reference to them in an underlying project file. Try deleting the project's .CPD file. If that doesn't work get your hands on a search tool and look through the project directory for the image file name. Let us know how you get on.
Thanks, well I moved all the old image files to another folder, and will hang on to them just incase something goes wrong, but it looks like everythings ok. I prefer to keep images in the same file as the topics they relate to, iamges that are used in several topics are usually in a snippet or something similar, so they are in another folder called snippet_images anyway.
I deleted my 75MB CPD file, and that started a few fun things. Random crash a couple times while saving after re-opening the project, some topics were not opened, so I manually re-imported them, and some images that were in use in the CSS didnt come through either.
but all sorted now and it looks to be fine.