Your images are of different file types. You have some images that are JPEGs while others are RAW. You need to make sure that all of the images are of the same file type. You can find out what the file type is simply by right clicking on the image (within the source folder) and choose properties. This will tell you which file type the image is.
Once you convert all the images to the same file type, you'll be good to go. Remember, you can convert all of the images to JPEG or, if you have RAW files, you can just use all of the RAW files. You can go from RAW to JPEG, but you cannot go from JPEG to RAW. By RAW, I mean any of the proprietary RAW file types such as Canon's CR2 along with the open file type DNG.
Personally, I set my camera to not use the proprietary file system which is PNF (Pentax's version of a RAW file). I set up my camera to save RAW files as DNGs. This is a much simpler file type to work with since all editing programs such as Lightroom, CaptureOne, OnOne, etc... can easily work with DNG files but not so much with the proprietary files types such as CR2, PNF, or NEF.