I use the term "pitfall picture" to describe, when a bird is difficult to id with one (or more) picture(s), because features visible are appearantly changed due to light conditions, angle of viewing, jpg-artefacts etc.
I will try to explain this with your Goldcrest (I hope this is...
welcome to birdforum!
Despite the description, that it looked bigger than a Willow Warbler, I think its a Goldcrest:
sleek, upright posture may be due to "alerted position" in a single picture.
-yellow crown borded by black (black area o the forehead burned out in a...