What is the best size and format to upload a profile image on Fluther?

I wanted to put up a Holiday image for my profile but am completely lost as to how to get it to look right. It looks great on my computer till I upload it.

I am having trouble getting my profile image to look clear. So far I have uploaded it as a jpg at 72 dpi 5” x 5” to 300dpi 4” x 4” and no matter what I do it looks blurry in thumbnail and hazy in full- view.

What is the best size and format for a profile image?