If you were going for blog looks like a pro blogger then go with the custom thumbnails, they look better. If you're not and you're strapped for time then any of your art looks good as the thumbnail so don't stress about it XD

Flowering water looks nice, though I tend to prefer freshly fallen flowers over picked ones XD Would not have guessed you didn't know what style to go for as that one looks like you knew exactly what you were doing.

LoL hmm maybe raiding area 51 wasn't the best idea :D