It's probably fine, this is what you are licensing your images to be used for.
for resell its only with EL right? and i only have SL sold so far
No, tickstock's right: on most (all?) sites book covers don't need ELs (unless the print run from the book is over a certain number, different on different sites, normally 250,000 or 500,000).
You could check the terms of all sites you sold that file on, but this question has come up several times, and I haven't seen mention of any RF sites that require ELs for book covers (I've had a few book covers myself, no ELs).
If you were selling RM, a book cover would typically earn you more than an image used inside.