Sorry for kick this topic up but I've had the same problem as described in the first post. It seems that, after a while, the nextgen gallery (or maybe the whole WordPress installation?) starts using more and more memory.
I tried allocating more memory to my hosting by uncommenting the line "// @ini_set('memory_limit', '128M');" in gd.thumbnail.inc.php. Unfortunately, this fix didn't work for me. I've also tried altering the memory_limit with .htaccess but then I get an error 500 right away.
I asked my host (one.com) and they told me that they don't allow users to change the memory_limit. To me, that sounds weird, since it's the first host that I've come across that limits their users to just 24M. But hee, at least they're cheap.
So, I still get an error with pictures over 600px wide and portrait oriented pictures almost always give the fatal error. And yes, even though it had been working fine in the past.
I love NextGEN, but I've really had it with this host! Is there a workaround or a way to do this while using less memory? Or should I consider changing hosting?