The first "img" expression results in an endless recursion loop, so something is wrong with the expression, and it will be halted once it reaches the 2000 recurse limit.
The second "img" expression also requires recursion that goes much deeper than the 2000 recurse limit. So this expression will be halted before it finishes as well.
The second expression does not work correctly, even when I allow it to go as deep as it wants. It finishes fairly quickly but the result is not correct, so something is still wrong with it.
Since I rarely receive reports about regular expressions crashing mIRC, it seems that very few users create regular expressions that require heavy recursion (at least not intentionally), so the 2000 recurse limit seems to be sufficient.
Have you tried testing your expression in another regular expression parser to see what happens?