Adobe Photoshop - This is where I make all my covers and the majority of my artwork for my publications. The quality is unparalleled, and your freedom as an artist to create is limited only by your knowledge of how to use Photoshop. Downside? It's expensive and hard to use. It took me quite a while to learn and, fifteen years after my first exposure, I'm still learning. Upside? Youtube is full of great tutorials that weren't available when I started (Sheesh. I feel old now.) It's also purchased on a monthly basis now instead of a great whopping one-time fee.
Or there's always Udemy. They run great promotions for $15 or you might be able to snag a free class -- 100 Bestselling Courses-Learn Photoshop, Web Design & Profitable Freelancing
Photoshop for Dummies - Hey, it never hurts.
Creative Cloud - This is Adobe's version of iCloud. It's a photo organization system. There's an entire sequence of Abobe products from Lightroom all the way up to Adobe Lightroom and Effects. You'll have to play around and see which one is right for your needs and skill level. It would take an entire course just on Adobe to break them all down.
Picmonkey - Hands down the best free site for quick, fun, yet effective photo-editing I've found. There's a premium package with more effects but I'm perfectly content with the free option.
StockSnap.io - Stunning free stock photos. Definitely the best I've found online without a subscription or pay/download fee. Search by keyword. The selection is limited but the overall quality brings me back again and again.
Pixabay - another free stockphoto website. The quality is varied. I've found most images here require additional photoshopping to make them suitable for publication. I do see a lot of these photos across the web in other publications though. So if you use an image, make sure to modify it somehow and make it "your own." No one wants to look like everyone else.
DeviantArt - I adore DeviantArt. It's a great place for artists and consumers to meet. Do not poach free art from this site. Many artists post their work here simply to share it - meaning you can't use it for free - and some sell their work or use it for portfolio purchases. There is a store, a stockphoto area (that usually requires attribution and has other rules governing use), and a forum where you can commission artwork. But don't go to DeviantArt hoping for freebies. That isn't the purpose of this site. Be prepared to pay your artists for the work they produce.
Flickr - Another photo sharing site. The quality is extremely mixed. So is the content. If you're looking for something very specific or niche, this is your site, but be prepared to Photoshop or edit like mad. Also, WARNING, take off the filters at your own risk. Yeah. Eyeballs. They can burn. Don't say I didn't warn you.