they were probably 'read only' or protected (lock icon)
use the menus to 'unprotect' images before individual deletion
Most cameras won't recognize images that were created with something other than the camera model being used. That's the case here (images came from scanning prints and were copied to the memory card).
So, the only way to delete them with the camera is to use a format from the camera menus (since the camera won't recognize the images during playback in order to delete them).
You could also use a PC to delete them (or use a PC to format the card, provided you made sure to use FAT as the file system type, since many models won't support Window's default 32 bit FAT).
It's safer to use the camera since it knows the format types it supports (and you also take any problems with the USB driver out the equation, as well as any problems with writes being cached by the operating system).
I format my memory cards before every use via a camera's menus, and always have.