Upload forms are unforgiving: 100 KB maximum, 200 KB maximum, and no explanation of how to get there. This tool takes the ceiling as the input. Name the size you need and it reduces the PNG until the file actually fits.
It works in the order that costs you the least. First the colour palette comes down step by step, since that is invisible on most graphics. Only if the smallest sensible palette still is not enough does it begin reducing the pixel dimensions — and it stops the moment the target is met, so you keep the largest version that fits.
What to expect at each target
A typical full-screen screenshot starts somewhere around 1–3 MB. Getting it under 500 KB or 200 KB usually costs nothing you can see — palette reduction alone does it. Under 100 KB the image often needs to be a little smaller in pixels too, and under 25 KB you should expect a visibly reduced image unless it was small to begin with.
If the target is impossible to hit without destroying the image, you get the closest result the ladder reached rather than an error, so you can decide whether it is good enough or whether the real answer is a different format.