I work with a lot of sample test cases that are visual. Is there any convenient way to include them in my Java source and link them in Javadocs, so my IDE can automatically
As you didn't show any sources, I can only do a glass-ball guess ...
For any files which are needed for documentation purposes, one should put them in a subdirectory named doc-files of your package directories. These will then be simply copied by Javadoc to the output directory. Then use a relative path in the element.
I'm not sure if your IDE's Javadoc renderer will do the same, but it is worth a try.