[Plugin: NextGEN Gallery] Problem: Thumbnail Creation

I have problem with thumbnail creation.
For some galleries it works with no errors.
But for some images it gives me an error, something like “Memory limit exceeded…”

I tried to create thumbnail manually by visiting manage gallery option but it works for some images and doesnt work for other images.

I have no idea how to rectify this or increase the memory limit.

mine is a wallpaper website.