I think rephrasing it slightly would help, as I'm not sure most people understand that uploading the cover is uploading the cache versions, not changing the actual epub cover.
"Upload the cached cover images used for thumbnails or the sleep/off cover on sending a book if the KT has already generated them. Simply modifying the cover on the epub will not make the KT generate them again. Useful if you've changed the covers since previous transfer to the KT."
Then the second option would be a subset of the first (only clickable if the first is checked), visually indented so it would be clear it can't work without the first.
"Send the cached thumbnails and sleep/off covers every time you send a book, regardless of whether these have already been generated. Avoids letting the KT generate covers, making them display faster the first time, but taking up a little more memory."