Board Thread:Support Requests - Getting Technical/@comment-43465048-20200404092547/@comment-9605025-20200405035810

Short answer:

32x32 non-animated image using JPG, PNG, GIF, ANI, or CUR as the file format

If you want it to work on Internet Explorer, it must be either ANI or CUR.

Longer answer:

As I said, the file type and image size requirements are different for different browsers.

For example, Internet Explorer only accepts CUR and ANI files. Most browsers additionally accept JPG, GIF, and PNG but not SVG, animated PNG, or animated GIF. Except for Firefox which accepts SVG.

There is a similar lack of standards with regards to the acceptable image size. While Firefox is capable of accepting images as large as 128x128, Mozilla (the company that makes Firefox) recommends using 32x32 to reasonable ensure maximum compatibility.