If the line is dithered then coloring may leave small gaps. One way around this would be to add a smooth thicker line background transparent png above the line that is being used for the coloring. That should hide any edges under the fatter line.
Not sure if that is a reason for the pixel image in the first place - it may just be a stylistic or file-size decision.
We create all the images with AI. If an image is too large, it gives us bad performance. So we resize the images to 1024x1024, and that works well. Can ZIM convert the image to SVG or vector format?