The Avatat functions appears not to work at all on OS X: I always get the error, that the Avatar may be max. 16k in size (why, btw??).
However, I downscaled a picuter at 14k and it still gets the same error message. I assume, this is due to the stupid Mac resource forks where a preview image is generated and added to the file size, which means, that the actual jpg is below 16k but including the resource fork it is maybe 60 k or something.
but the point is: there is no (easy) way to avoid this on the Mac, hence this feature is not working at all…
the maximum size for an avatar is 8 kB - Spark allows 16 kB which is not in sync with the JEPs.
The recommended resolution is square and 64x64 pixel² while 32x32 px² is the minimum and 96x96 px² the maximum allowed resolution. Spark does not check the image resolution yet.