first off, let's start with your profile —
your profile is something that millions of people can view and serves as a first impression. you want to make it look satisfying, which makes people want to follow you.
1. have a theme.
for example, your icon and cover picture can be of the same colour scheme or the same subject matter/idea. the aim is to make them complement each other.
2. avoid using low-quality pictures.
ensure that your images are of high quality; otherwise, they will appear unattractive and as though little work was put into your account.
(there's a difference between grain and low quality)
3. for your icon :
try to use a picture with only one thing/person — it looks 'neater'.
TIP: you can edit a border over your icon in a circle to give a more finished look!
4. for your cover picture :
try to use images of locations or backgrounds as opposed to images of people or quotations, as your icon will obscure it. a backdrop of a single colour in the same colour scheme as your icon or overall account theme might also suffice.
5. don't use very 'common' pictures.
some users may even remember your account through recognition of your icon. fortunately, there are many niche aesthetics nowadays, so it shouldn't be too difficult to select an original profile photo.
TIP: tumblr and pinterest have countless of face cards, some of which even have a matching header, so look there! i also recommend browsing further into the hashtag to pick your photo, rather than selecting one of the first handful.