Thumbnails in Nextgen-Gallery

I am using Nextgen Gallery but I have a problem with my thumbnails. Look on my site : my site
As you can see, my images haven’t got any thumbnail and they look uggly. I tried to find what was wrong and I noticed that there wasn’t any image on my menu, whereas on this one, for exemple :
If I try to create thumbnail, I receive the following error :
“Following images can’t be created : (Error : Exceed Memory limit. Require : 28.15 MByte)”

Please help me ! This problem makes me angry. xD