@Eddie Davidson
like she is about to crack a joke or she just farted and nobody knows it
I like those - either or both! Funny, and they get to what is important - what's inside her, especially if, in the scene, she really is in a playful or mischievous mood.
I guess one question is the purpose of your description. If it is to help make a character memorable so the readers goes, "Which blonde? Oh, yeah, the one with the lopsided smile," then a short, easily-repeatable and recognizable description, such as lopsided, might work best (especially if that expression is indicative of her general emotional state, see below). If the description of an expression is to indicate an emotional state, then I think it best to describe then name the emotional state (if you're writing from the POV of the smiling character) or what the POV character judges the emotional state of the smiling character to be. It is impossible to convey internal emotional states by descriptions of external expressions. ("Did you see Tom; he looked sad. Bet he's missing Jane." "Oh? I thought he looked pensive. Probably thinking about what his options are now." - Two different characters observing the same expression and reaching different conclusions, something that happens IRL all the time.)
For the second picture, I agree with the comment already made that the smile does not reach her eyes. It is not a real smile. In psychology, a smile that reaches the eyes (crinkles the skin around the eyes due to activation of small muscles) is called a "Duschenne smile." In one interesting study, women who were able to produce a Duschenne smile for their senior picture in their college yearbook were more likely to be satisfied with their marriages in mid-life.
The replicability crisis in psychology, medicine and other fields should raise yellow flags for any point backed by one study. However, there's been further research on this and the idea that a Duchenne smile indicates authentic positive emotions most times in natural situations seems pretty solid, although there are cultural components. A recent literature review done, in part, by a researcher I respect indicates that such smiles are a good marker of a chronic positive mood, and CPM correlates with greater success in love and work, as well as better health, longevity, etc. Thus, if an author wrote, "His frequent smile - the kind that re-shaped his entire face into a poster for happiness - reflected his fundamentally positive stance toward the world, was what drew her to him most strongly," it should ring true to most readers because it would fit what they intuitively believe.
