To know what size and resolution (quality) the picture should be, ask your printer or look on the online on their websites. In general, you need 300ppi but if you use a vector, there's no resolution to worry about. The resolution of the picture depends on the method you'll use. Since you want to print on t-shirt, you can use a lower quality image and cheat a bit on the size transfer on t-shirt isn't the same quality as offset printing on paper for example. When you look at a picture online, you need to imagine that this picture can usually be used for a printing project 3-4 times smaller than what you see on your web browser. Web pictures often don't have the right amount of pixels to be used for large prints. Find stock picture websites, read the terms and licenses, take your own pictures or create your own illustrations for this. Even 'free pictures' online cannot be trusted because some websites redistribute the work of others without permission or offer very limited permissions on how to use them. In general, don't use Google search to find pictures if you plan to print it on a product and to sell it. If you plan on selling a t-shirt with the face of a celebrity on it, you can't! You need to get permission for this or buy commercial rights for such a picture.

Shirt illustrator