I have the same problem. My photo size is just 8kb.
It's not the file size, it's the "pixel size" (the number of pixels, horizontally x vertically).
Make sure that this "pixel size" is about 80 x 80 pixels. Use a graphics program or an online service as mentioned before.
To check the current number of pixels in Windows Explorer, right-click on your picture file and select "Properties > Details".
P.S.
On The problem that an avatar is not accepted is usually not the file size, but the number of pixels.
With 80*80 pixels it certainly works. Normally up to 90 pixels are possible. I recommend .jpg as the file format. But it should also work with .png or .gif, for example.
I suspect your picture is (much) larger than the number of pixels mentioned. Please try to resize the picture to around 80*80, maximum 90*90 pixels. You can do this in a suitable photo editor program or on a website like >>>
picresize.com:
Best regards,
Chris
Edit: Link changed