Hey Arnold! has a great, if not rabbid, amount of fan drawings. These can be sketches, paintings, coloring, and even made in computer paint programs.
Here is a page for artwork done over the years.
DISCLAIMER: DO NOT STEAL OR COPY FANART FROM THIS PAGE!! THIS IS AN ARCHIVE SHOWCASE OF FANART OVER THE YEARS AND ARTIST CREDITS WILL BE CREDITED WITH THE PICTURE TO PREVENT PLAGIARISM, THEFT, GIVE PROPER CREDIT, AND/OR CONFUSION.
Arnold w/ basketball
Arnold in front of Future Tech Industries
Arnold & his mother, Stella (drawn by latinart)
Arnold drawing (drawn by lampii)
Arnold Sketch (drawn by artpassion88)
Colored Fan Art (drawn by latinart)
Many Arnolds (drawn by latinart)
"Something has gone wrong" (drawn by merrymarmalade)