This is a small selection of graphic editors and utilities for some of the platforms allowed on the competition.
Atari
Atari Interlaced Studio 1.4.7 (Windows): Graphic editor with support for many graphic modes, including interlaced ones. Can export as executable files (.XEX). Link
Apple
Ghetto Image Converter (PC): PPM to HGR2 image converter, requires a PC with lua 5.1. Link
Commodore
NUFLI editor (C-64): NUFLI editor. Link
Project One (Windows): Editor/Converter for Hires/Multicolor/AFLI/FLI/IFLI/Drazlace modes. Link
GangEd (Windows): Very basic Hires/Multicolor & semigraphic modes editor. Can export Hires/Multicolor as executables. Link
ConGo (Windows): Image viewer/converter, can export Hires/Multicolor/IFLI/FLI/MCI as executables. Link
MSX
MSX Paint IV (MSX2): Screen 5, 6, 7 y 8 editor. Requires mouse. Link
piroPaint 9918 (Windows): Screen 2 editor. Link
MSX Screen Conversor (Web): PNG/GIF/JPEG to Screen 2, 5, 6, 7, 8 y 12 converter. Link
Sinclair
OCP Art Studio (Spectrum 48K): Complete graphic editor. Link
ZXpaintyONE (Web): ZX-81 semigraphics editor, written in HTML5. Link
ZX-Paintbrush (Windows): SCREEN$ editor for the ZX-Spectrum. Link