Popular and prolific game asset creator Kenney has a page up at Github that links to some of his favorite tools for manipulating pixel art, such as creating sprite sheets and extracting images out of them. It’s got a lot of useful info! If you have an interest in this style of art, especially making games with it, these programs are worth having in your toolbox.
To give to some incentive to click through, some of the projects linked are:
A Spectrum emulator written in Rust, and another one in JavaScript
A port of Spectrum game The Great Escape to C and current platforms
Tools for working with archives of Spectrum cassette tape images, including to convert one to mp3 to facilitate transferring to a tape for play on a physical ZX Spectrum
A Visual Studio Code extension for working with Z80 code
And small program to convert image files to the peculiar limitations of the Spectrum’s graphics hardware, to give your portraits some of that loading screen flair.
That last one I tested out a bit, here’s some results. I discovered it’s best if you resize the images to around the Speccy’s 256×192 resolution before processing.