I use Coffeecup HTML editor and one, just one, of the extras they provide is a heap of graphics from Corel which can be used for ebooks too.
My next book will have many graphics from Corel.com - I bought a couple of their packages and the range/value of the included graphics was very good.
I was a member of ArtToday http://www.arttoday.com/ for a while. They provide great graphics too.
For graphics software, try PaintShop Pro from http://www.jasc.com/ and you probably won't need anything else! I am still using PSP version 6 while version 7 is the current one.
This is because I bought PhotoImpact from http://www.ulead.com/ and they provided a range of supplementary utilities and heaps of great graphics including an extra Megabyte of templates which came available for download each month.
I am still using the previous version of that too because it does everything I need. I can confidently recommend the current package because I have seen some great stuff produced with it.
That brings up another point (then I'll stop, I promise!) both these graphics packages have free supporting newsletters and forums run by users which are very, very good.