best photo app to fade a picture

Do you mean to make it less bold? I believe photoshop is free and may allow you to alter the opacity. If you want to make an image lighter you put a white background on a layer, the photo 'over' it and make the photo less opaque....as in lower the opacity.

If you want to make it darker, then you use a black layer on the bottom

One of my favorite ones for special effects including opacity, I use Swift Publisher 4. It does many other things - the opaque function is just one of the best I've encountered, but: it is NOT a photo editing app as such.

Since you do not tell us your device, OS, or what "fade" means - I suggest you do it online at https://imgur.com