You might consider getting irfanview, a free program that is small, that has batch conversion options, including rename and resize. Resize is under advanced options under batch conversion.
It is much simpler to use than some of the other programs mentioned. The others are great for more complicated work.
You may need to decide who your audience is for the book. The images needed for readers like your Nook can be much small and simpler than for ipad and computer users. Change color depth is an advanced option under Batch Conversion in irfanview.
Images can be saving using varying levels of color information from only black and white, 16 to 256 levels of gray, 256 different colors up to millions of colors. The e-ink readers can only show 16 levels of gray so any other colors have to be converted by the reader anyway.
Make sure you have your original epub safely away on a CD or DVD while you work with this or rename the file while you work with it. You may spend a while at this if things don't go well and something may seem like a brilliant idea after a number of hours, but instead wipe out all your work. If you use Sigil and work with the code, it is possible to get in a situation where you leave out a closing command, Sigil will offer to fix it automatically and automatically leave out a good chuck of your document.
But if things go well, it can be as simple as dwig outlines.