questions about uploading downloaded art from this site

questions about uploading downloaded art from this site

Hi, i'm new to this site and i am trying to use tiled to create maps for my game and i have been directed to this site as it it a good resourse for what i've been looking for. However when i attempt to import the art that i downloaded into the tile sheet on the program it fails to find the sheet. I was wondering if someone has the answer to my problem or can show me where i can find the answer. Additionally if someone could tell me what i need to look for so that the art i download will be compatiable with tiled that would be most helpful.

Thank you so much for any help you can offer.