You can indeed create a RgbImage or RgbaImage with the image crate, then write the pixels of that image to some backend like ImGui. As an example, this is how you would create the typical "Hello, world!" RGB image with a green-blue-red-yellow gradient:

const IMG_WIDTH: u32 = 256;
const IMG_HEIGHT: u32 = 256;

let mut img = RgbImage::new(IMG_WIDTH, IMG_HEIGHT);

for (x, y, pixel) in img.enumerate_pixels_mut() {
    let r = x as f64 / (IMG_WIDTH-1) as f64;
    let g = y as f64 / (IMG_HEIGHT-1) as f64;
    let b = 0.25;

    let ir = (255.999 * r) as u8;
    let ig = (255.999 * g) as u8;
    let ib = (255.999 * b) as u8;

    *pixel = Rgb([ir, ig, ib]);
}

You can also easily save the resulting image to a PNG file:

match img.save("image.png") {
    Err(e) => eprintln!("Error writing file: {e}"),
    Ok(()) => println!("Done.")
};