Open a PNG in your browser and save it as an .ase file that Aseprite opens, keeping layers and frames if you build them. No account required.
Open BitRasterFree, runs in the browser, no account needed.
The .ase is written to Aseprite’s own format rather than something that merely imports into it, so layers and frames arrive as layers and frames. A flat PNG in gives a single-layer, single-frame file out, which is the honest result.