<p>Not a complete list.</p>

<p>These definitely work:</p>

<p>PNG, JPEG, GIF</p>

<p>examples of the first two are easy to locate; the third can be done, as shown <a href="https://meta.stackoverflow.com/a/369254/330679">here</a> for example, or on our own site <a href="https://stats.stackexchange.com/questions/153726/proof-of-algebraic-formula-for-the-sum-of-two-dice-toss-as-a-convolution/153775#153775">here</a>.</p>

<p><s>Pretty sure these won't work (currently at least)</s></p>

<p><s>SVG</s></p>

<p>I haven't spotted a bmp or a tiff yet (and not certain stack.imgur.com supports those), but there might be ways to do them. However, the fact that on stackoverflow many posts discuss BMP files but display PNG files when talking about them (e.g. like <a href="https://stackoverflow.com/questions/39823509/inverting-colors-of-pixels-in-a-bmp-file-in-c">this</a>) suggests that maybe BMP files are either not possible or may be possible but might be actively discouraged (if so, presumably for space reasons); if that's the case TIFF files would have similar issues.</p>
