The LZW patent (that gifs use) has been copyrighted and any program that wishes to provide any kind of gif support must pay a quite large amount of money to the company that owns the patent. Read more details here. (thanks qwerty ).
The LZW Algorithm Patent will expire to public domain before the next version of mIRC is released (June 20th, 16 days...). :P
I'm also reading a number of websites that say "Many legal experts have concluded that the patent does not cover devices that can only uncompress LZW data and cannot compress it." That means that if mIRC only reads the .gif file, it should be safe from legal issues.
If you read on burnallgifs.org it says it expires on the 20th in the US, mIRC is not made in the US. It says the patents last longer in other countries.