I am not too sure about this, but as I understand this the icon needs to be in a pix_core directory so that it can be overwritten by a theme icon, because $OUTPUT->pix_url('i/info') will take care of that, so your html writer code might look like this...
Is this info.png the same info.png that is in Moodle already or a special one? If it is in Moodle you don't need a copy of your custom block image, also I think pix_plugins/block/my_account_lite/info.png is wrong as it should be in pix_core/i/info.png this way it would be over written by a theme icon.
I hope this helps?
By the way, you might be better asking this question in the General Developers forum. There are lots of people understand PHP better than me.