So, disclaimer first: I haven't as yet actually self-published on Amazon but I did a fair bit of research a few months back and I have my self-pub bible open on my desk (Catherine Ryan Howard's
Self-Printed - it's the single most useful book on the subject I found and blissfully bereft of unfamiliar jargon). If there are any errors here, hopefully the other kind folks will correct me...
the cover. How flexible are you allowed to be?
If you're publishing via Amazon then there you have 2 different options - kindle e-book publishing, and paperback publishing via Createspace. You can publish the same novel through both, but bear in mind they are essentially different products as far as the publishing process is concerned (at least in terms of the cover).
For kindle, you upload a cover which would be equivalent to your front paperback cover. Amazon provide minimum dimensions, file specifications (JPEG/TIFF) and and file size limitations:
Amazon Kindle Direct Publishing: Get help with self-publishing your book to Amazon's Kindle Store
Essentially you should keep the height/width of the image to a 1:6 ratio and follow the other suggestions for the highest quality image you can get within the file size limit. Within those specifications you have a fair degree of flexibility (you can lay out the cover design/picture how you like, place the author name & book title where you like). If your cover image meets Amazon's requirements you can upload it.
Those 2 cover images you linked to in your 2nd post? I'd say it's a design issue, one was a modern professional design and layout that probably cost a fair chunk of cash and the other looked like an old paperback cover from a classic (which I think was the intention). Cover design now is a whole cottage industry and if you want a complicated but arresting cover image you might be best advised to approach a cover designer (it also cuts out some of the frustration that comes with doing it yourself). People can and do create their own covers but there's a wide range of quality in them, and if you want something really good it's probably going to take a while to get right, at least if it's your first go.
Publishing with Createspace is different again: you upload the front cover, spine, and back cover as a single image so it's on whoever's designing it to get the layout right with the spine sitting in the right space (which will depend on how many pages your novel runs to - Createspace has a calculator for working it out). I seem to recall you upload it as a PDF but I could be massively wrong on that. The dimensions will also depend on what height/width you choose for your paperback (there are various options). It's pretty mind-boggling, right?
Can I upload a cover image which includes the text?
Yes. The uploaded image for both kindle and paperback must have the text within it (Amazon don't add it for you) so once you've got the actual image you need to edit that image and overlay the text on top. There are free image editors and the like out there on the web that you can use - a friend uses something called Gimp, but I've never tried it myself.
There are a number of reputable companies out there that do pretty affordable covers (e-book and/or paperback) so in terms of time-saving that might be your best option. For low cost covers check out the list put out by Smashwords:
Smashwords – Mark’s List
As I haven't done much more than stick my toe in the pool you should probably take the above with a pinch of salt (and hopefully someone will correct any errors I've made). Best of luck whichever option you choose.