You can remove one or more images from a collection by selecting the image thumbnails in the Collection Viewer, and then either pressing [Del] or right clicking on a selected thumbnail and selecting “Delete selected images” from the context menu. IDimager will then ask for a confirmation.
An important choice here is if you want to delete the original images from disk as well. By de-selecting this option you will not only delete the collection-image relationship, but also the image from disk. So please use it with caution. (Note: this option is always disabled when removing images from a Media Explorer collection. This is because collections in the Media Explorer display the contents of the physical folder, so removing an image from a Media Folder collection will also remove the image from the disk.)
By default, IDimager will delete images from disk and put them in the Windows Recycle Bin. That way you can always recover the deleted images or flush the Recycle Bin once you are sure you no longer need the image. You can disable this setting in the options dialog (Tools -> Options -> General -> Deleted images go to the Windows Recycle Bin), but it is recommended to keep this option switched on.
One caveat in this respect is that Windows does not send deleted images to the recycle bin when these images are stored on a network drive. When you access images from a network share, please be aware of this behaviour by Windows.