Image upload difficulties

Have just tried to upload 7 images into one gallery but was informed that only 3 were actually uploaded.
On uploaded gallery list three have thumbnail pics but the other four just show clipart of a ?. Yet when I click on this symbol the relevant image does appear.
I have also noticed that the photo’s successfully uploaded have the date when created/edited with a pixel number, but the unsuccessful ones show a date of 1 January 1970 and a pixel number of ‘x’
Any advice on how to resolve this issue greatly appreciated

http://ww.wp.xz.cn/plugins/nextgen-gallery/