Theme Evolve not working right with Nextgen Gallery

Hi everybody,
I have problems when uploading images in the Nextgen Gallery. I use the Evolve theme.
I can get the image only in one width. When I upload an image, say in 900 x 750, it is uploaded till maybe 600 x 450 or something. Somehow it is reduced to a lesser size format and I have no idea why that happens.
I already tried to get an answer from the theme author(Evolve theme), but I think he doesn’t know, since he doesn’t answer this question.
I also asked this question in the Nextgen Gallery forum, but no answer there too.
This is the site of the Evolve Theme:

http://theme4press.com/evolve/

And this is my website with the magazines:

http://www.secretsistersblog.com/magazines/

What can I do?
Johanna

Update

I investigated a little further. I installed the WP Easy Gallery and that Gallery let me upload the images in the larger format. So it is definately the combination of the Evolve theme and the Nextgen Gallery.
The WP Easy Gallery though isn’t that easy at all, so I would prefer to use the Nextgen Gallery.
Somebody has a clue what to do?

Johanna