Honestly, I'd refrain from using original artwork from someone else or copyrighted images from Google, unless you have permission from the original author or give at very least some credit (i.e. "illustration on cover by X"). There are plenty of websites where you can find free stock photos (like Pixabay, as mentioned above) and there are free softwares (Gimp, Medibang, Krita + countless free websites) that allow you to edit pictures and add text, so there's really no reason to steal. If you own Photoshop and have a few coins to spare, you could also search GraphicRiver to look for book cover templates or illustrations: you can get one for prices as low as 4$. Or you could commission an artist as you did!
Anyway, I don't see the issue with using other people's artwork as a cover for novels (provided that credit is given and that the art is not stolen, as mentioned above XD).
For comics... eh, not really sure about that. I mean, if I had an already established series and people were already familiar with my style, I could maybe have a "guest artist" doing a "bonus cover/illustration" or something like that to feature in one of my episodes. But as for using it as my official cover art on Tapas? I doubt I'd do it, as I think it'd be confusing for new readers and probably drive them away once they realize that the style doesn't match.