there are many websites that do not support upper case letters, so when i give a imgur link the users can't see the picture. i wish imgur would give only lower case links.

Uppercase letters are completely valid in URLs. In fact, most websites have case-sensitive URLs - including Imgur.

Allowing uppercase letters means that there are many many more filename permutations which means we won't have to extend the length of our filenames for a long time.

There are no plans to make the URLs lowercase. This is not a problem with Imgur, it's a problem with sites that ignore uppercase filenames.