Modern photo editors (GNU Gimp, Adobe Photoshop, Zoner Photo Studio) are usually native apps, which have to be downloaded and installed on the device. It takes a pretty long time to turn them on and off. 

Photopea.com is a free online tool for editing raster and vector graphics with support for PSD, AI, and Sketch files.

It is one of the best Adobe Photoshop alternatives for me.

Supported Formats

• Complex: PSD, AI, XCF, Sketch, XD, FIG, PXD, CDR, SVG, EPS, PDF, PDN, WMF, EMF.
• Raster: PNG (APNG), JPG, GIF, WebP, ICO, BMP, PPM/PGM/PBM, TIFF, DDS, IFF, TGA.
• Raw: DNG, NEF, CR2, ARW, RAF, GPR, 3FR, FFF.

https://www.photopea.com/