Add thick outline to existing artwork in Illustrator [duplicate]

Question

I have some vector images they I need to add a thick outline similar to the image below.

Is this possible to do quickly and easily in Illustrator?

Note : I have permission/licence to edit and publish the images.

Answer 1

The easiest way is to use the Appearance panel and add a stroke to either the grouped artwork or the layer that it is on; there is no need to flatten or duplicate anything then and the stroke will update with any changes to your artwork.

Use the button at the bottom of the panel to add the stroke then drag that stroke below "Contents" (otherwise it will apply the stroke to everything).

Take this handsome fellow for example:

The same artwork with a stroke added to the group:

Answer 2

The key here is to set up the vector so the stroke doesn't interfere with the integrity of the artwork. Start with a simple Copy/Paste in Place (Command + C, Command + F) on the original image to preserve any details. Hide that top graphic (Command + 3) and make the bottom graphic one block of color (same fill and add a stroke if it's not already there). Once you have the stroke to the desired thickness, make the hidden image visible again (Command + Option + 3) to see results. If the stroke isn't where you want it to be, simply repeat hiding the preserved graphic and adjusting the stroke until you achieve the desired look you're going for.

That being said, please be sure to carefully check the license on the graphics from wherever you downloaded them to ensure that you can indeed make revisions or changes to the artwork.