The issue designers run into when designing for published works, is finding good images that work for the feel of the cover. If you aren't an artist then it's sometimes difficult to find the image you want.
With Wattpad covers it's a bit easier. I hate to disappoint all these new designers who think you have to use royalty-free images, you really don't. The 'free' sites that offer images often don't have a great selection, it's even more limited than stock sites are. I do use them, but not a lot. Many of my images come from Pinterest or Deviantart, sometimes just random Google searches. For people here to insist you need royalty-free images is ridiculous, especially when more than half the books on the site are fanfictions and people are using the actors/models/singers/or whatever on the cover. Technically you're using their image without permission and that is a violation. So why are you worrying about where the background images came from?
Most copyright holders aren't going to worry about Wattpad unless the use of the image is so egregious it damages them in some way.
There are several royalty-free sites, yes the selection of images is limited, but they do help. Here are some sites for you -
stocksnap - stock snap is part of Shutterstock which is a paid site, so generally the top images (with the stars in the corners) are Shutterstock images and you have to scroll down past those for the free ones. There are times when the Shutterstock images are all that's available. Their selection seems very, very limited for the free images.
unsplash - this site has a decent selection and no push for a paid site.
pixabay - this site has a decent selection, but they also put a selection of images across the top of the page for istock which is a paid site. Though their push for istock seems less intrusive than stocksnap's push for shutterstock since they just put a 'ribbon' across the top of the page with the selection of paid images, rather than adding it to your actual search as stocksnap does.
pexels - this site has a good selection of free images and I use them quite a bit. They also have a small push for Stock but it's minimal and consists of an image that's about the same size as the offered free images. It's essentially a box you can click on to go to iStock.
The images on these sites can be used for just about anything graphic-related. Do check to make certain the author of the image/s has what they offer free for commercial use if that's what you're planning on using it for. Not all of these sites allow images to be used for commercial work, like published book covers. In fact, a lot of graphic designers don't recognize these sites as a place to get cover images from and if you belong to any cover groups they'll boot you for using these sites.
You can also check out DeviantArt, there are artists and photographers on there who sometimes have free downloads, again check their rules to be certain you can use the image for what you want. faestock has awesome images. Hers are great because she often takes a lot of different shots in the same outfit in different poses. She also sells her images on Depositphotos, I have purchased a lot of her images. HWWOStock has great stuff too, but they charge for everything, though I've found they usually run sales in their Etsy shop and you can get stuff fairly cheaply.
Some places will offer royalty-free images but they can't be used for commercial purposes, they're intended for personal use and not to make money off of.
So be aware of that and do check licensing before you commit to using an image for anything where you will be making money. Not all royalty-free sites allow the images to be used for monetary gain. Some allow it but the images must be significantly altered to use it (which means you can't just slap text on it and use it). You also can't use images if there is an easily recognizable logo, product, or person in the image, the license on the free sites does not cover this, and permission for use must come from the trademark holder or person themselves, not the photographer.
You must be aware of the licensing on the paid sites as well. Most designers buy the standard license, which covers basic uses. An extended license is needed for wider distribution. Like if you're designing products for the image to be used on (key chains, for example). The license is also granted to the purchaser and cannot be transferred, which means you cannot provide the original image to whoever you designed the graphic for. You can only provide them with the final design. You cannot provide PSD files either since it contains the raw image.