Audiotool board archive

Personal go-to for adding transparency and overlaying: http://www.online-image-editor.com/

If you use an IPhone, there's an app called "TAD" for this purpose: http://tadtheapp.com/

Another option I use are the PIXLR free online image editors. I recommend you "O Matic" for quick nice effects on a pre-existing image: https://pixlr.com/web

If you're interested in generative art and know how to code a little, then you could use: https://processing.org/ :)

This one works nicely! https://www.befunky.com/es/opciones/editor-de-fotos/

I don't know if you want to go to this much trouble, but I find one of the best ways to make logos and art is to use dedicated graphics software. I personally use inkscape:
https://inkscape.org/en/
It can import pictures just like any photo editor, but it's particularly useful if you want to make your own covers from scratch using computer graphics. If you're not into all that you can just download a basic photo editor, such as GIMP (It's free): https://www.gimp.org/

I use Canva, you can look for either the browser version ir the phone app

i use sketch.io it is a free online site, and it is really easy to use.

I use good free editors, i found here https://www.cevagraf.coop/blog/editor-de-fotos-gratuito-para-retoques-fotograficos/
They have a review of various. I hope they can help you.