Cover Art
It is hard working with basic graphic programs I'll admit, but it's harder to accept someone else's art/illustrations . I know several people who lined up what they wanted for the cover of their book, art concepts that actually gave an indication of what the book was about, grabbed attention, touched the essential senses etc etc only to have a publisher's in-house marketing art director insert some vapid creation of their own. My mantra is : if the 'thing' is going to fail, let it be entirely on my head ' and then there is no one else to blame or be frustrated with but yourself (and we know that doesn't last :) )
Try to be as original as possible (I use my own photo images where necessary and can claim copyright) and look at original art work as just part of the creative package for that book. Don't overlook logo developement either, and of course , as mentioned last year, your own website. If you need to clarify location in the book, don't be afraid to include an aerial or 'Bird's-eye view' plan of where the action takes place, only make it easy to read or it'll be skipped. If an object is integral to the story, perhaps a clue in itself, feel free to draw free-hand but present it professionally.
Writing and Artwork go together like a 'horse and carriage'. You had the idea(s), now caption it/them in an illustration out of your own head--- the satisfaction of having crafted the complete package is well worth the effort.