Thank you for reporting this problem.
Unfortunately, this is actually a known issue which the developers are aware of. As such, it should hopefully be fixed in a future version (although I do not know when this might be).

Essentially, the 'Gallery -> Open...' option from the drop-down menu at the top does not currently work from the 'Images' or 'Customize' tabs.
If you want to open a new gallery without closing and restarting JuiceboxBuilder, just click on the 'Start' tab first and you will then be able to open a new gallery via the 'Open Gallery...' button or the 'Gallery -> Open...' option from the drop-down menu at the top.

I hope this workaround is not too much of an inconvenience until the bug is fixed.
Thanks again for reporting the issue, though. We appreciate the feedback!

I read on a post, to import the photos must not exceed 1GO

Adobe's AIR's maximum memory allocation does not exactly equate to the total filesize of the images you add to your gallery in a single batch.
As you have discovered, a problem could occur with the total filesize of a batch of images being much less than 1GB.
If you find you have a problem adding many images at once, then trial and error should help you to determine a safe number of images per batch. (It will depend on your own images so I cannot recommend a maximum number of images which will work for all users.)
In any case, this is another known issue which the developers are aware of but if, as I expect, the problem is due to a limitation within Adobe AIR (the platform on which JuiceboxBuilder runs), then there may be little that can be done about it.
At least you are aware of the workaround (to add images in smaller batches).