Wanted to copy a photo to my k750i to use as wallpaper. I used the USB drive feature to copy the photo to d:\DCIM\100MSDCF where the other camera photos are but it refused to show up in the 'Pictures' explorer.
I eventually got it to list by renaming it in the same pattern as the existing files, DSC00025.JPG. Then I could copy it into the phone memory.
Twitterings

Yeah, um, in that situation i simply copied the picture i wanted to put as a wallpaper into the "other" folder within the system files. Then you can access the picture with the file explorer on the K750i...