photopea
Online free professional full photo editor (like Photoshop).

piskel [online]
free and open source sprite maker

tinypng
compress images

pinetools
multi image edit tools: resize, crop, remove bg, pixelate, flip, editing in bulk & more

resize-image
image resizer

dither me this
dithering images with many presets

ditherit!
dithering image basic (some presets)

doodad
Multi-format Dither image editor and compressor.