For the old Facebook timeline layout, you can check out these best 3 websites to customize your profile pictures, as mentioned before.
And for the new layout, you can customize your Facebook timeline cover with the following 4 different ways:
1. Design a photo yourself
The size of the photo for the Facebook timeline cover is 851px*315px, so you can design a photo in that size and upload it as the cover.
2. Choose a third-party photo
Among the above 10 websites, some of which will upload your selected photo to your Facebook albums, while the others require you to download and upload manually, and when the photo is uploaded, you can use it as your timeline cover.
The photos are free, but there will be website marks in them, or websites URLs posted on your timeline when they are uploaded.
3. Use your Instagram photos
And you can also only use the photos in a particular category, such as Flower, Girl, Pets, etc., or with a particular tag. For the finished photo, there will be a InstaCover badge in the bottom right corner.
4. Use your Facebook photos
For the finished photo, there will be a Pic Scatter mark with some text in the left bottom.
As what you can see from the above, the most professional and personal way to customize your Facebook timeline cover is to design a 851px*315px photo yourself, to do so, you can take a photo, and edit it with some online image editors.