Don't store your pictures on your desktop. That is about the worst place to store them, both from an organisational point of view as from a performance point of view. How do you think you can handle 300+ pictures? You would not be able to find PR, let alone import or edit those stills.
Get organized. Store them on your stills disk, probably your E or F drive.
Your workflow does not make much sense. Why unlink a portion of your clip when you can simply overlay the still on a higher track.
I suggest you start reading the help file and manual, watch tutorials and read some books on video editing. Classroom in a Book comes to mind.
I agree with Harm on the setup of your Project. In very general terms, I will create a folder for each of my Projects - one per Project. Beneath this "root" folder, I always create Still Images*, Captures, Video, Audio, Music, SFX, Artwork, etc. If I am working with existing Assets, Copies of these are added to the appropriate folders, and the originals are kept on my archival storage, like my NAS. The exception to this is my Captures from tape, which are the original Captured files, but then I never recycle my tapes, so I could always re-Capture, should I need to. It is all right there.
In PrPro, I create Bins with those same names, plus Titles and Timelines. When I Import, it's into the appropriate Bin. When I create (Timelines and Titles) they are created in those Bins.
I also like Harm's suggestion to Overlay your Video with the still images, and leave the Audio untouched.
Good luck, and hope that this helps,
* for larger jobs, I might have sub-sub-folders for say Still Images>1900-1926 & then 1926-1958, or I might create sub-sub-folders by maybe Sequence name.