I'd suggest using the media library for your images. In the Excel sheet, if you have at least the name of the image stored, you can concatincate the actual URL based on the file placmeent in the media library.
Create the media library in Kentico. Find the directory in the file system. Typically under
/cms/<sitecodename>/media/<medialibrarycodename>. Example for
MySite with library named
images would be
Next, copy the files into that directory in the file system:
Then, in you're Excel sheet, update the filename to prepend
/MySite/media/images/ to the file name. This will get you the relative link you're looking for in that document.
Lastly, you'll need to go into the media libaray and import all the images.