@Eddie DavidsonThe problem with what you're planning, is that you'll likely 'borrow' copyrighted material, and will open yourself to lawsuits if anyone recognizes their material in your posts. Chances are, you can likely get away with it, but it's not a wise move, either for your reputation or from a ethical standpoint (most on this forum are understandably against copyright violations, for obvious reasons).
To put this into context, I've always had a knack for very realistic freehand sketches, yet aside from my latest book, I've never attempted to illustrate them with my own work. Instead, I use Stock images, where I legally purchase the images I use to illustrate my story, so the ownership is free and clear. Yet even there, I've had several purchased images vanish from the Stock Image library, as someone reported the images as copyrighted by someone else, and rather than admit it, they simply remove the images, expected users to 'notice' it's no longer available and simply stop using them on their own.
There have been a few authors who've simply 'requested' images on photo sharing sites (i.e. DeviantArt). I tried that myself, and could never get a single reply, so he either had an 'in' or had a previous relationship with the artists.
As far as best software, there are various products (the only one's I've noticed are on iPhone and iPads) where you can add elements to images you already own the rights to (thought and speech balloons, for example). Those seem easier than attempting to master a complicated software for 'managing' artwork, as most of the images end up with disjointed limbs (ex: limbs extending from their chest, or hands disappearing into their thighs), which therefore appear amateurish.