If you want to fly your freak flag in Valheim, there's now a mod for that. The Custom Sails mod lets you dress up your drab Viking ship by slapping your favorite image onto your sail.
And it's super easy, too. With the mod installed, interact with your ship using the Left-Control key. A pane will pop up, and you can paste in the URL of the image you want to use.
Please, please, make it a nice image. Or a funny image. Or a cool image. Please don't make it a terrible image.
Anyway. Once you've entered the image URL, it'll appear on your ship's sail, just like that. Naturally, I went with the iconic image of Geralt in his tub, but I'm sure there are plenty more (nice, funny, cool, and not terrible) images that would be perfect for your Viking longship.
How do you get the image off your sail? Well, I tried deleting the URL from the entry pane but it didn't work. I deleted the mod itself and got my old familiar sail back, so that may be the only way.
The mod is client-side, so you'll be the only one who can see your custom sail, unless the people you're playing with also have the mod installed. The Custom Sails mod requires BepInExPack Valheim to run: once you've got that installed, simply drop the CustomSails.dll into the plugin folder.