Thanks for these informations Joachim and sorry for not having read carefully the board rules.

Maybe I needed to clear out the purpose of my gallery. Registered users need to be able to download my original images (max resolution), therefore, they are over 5mb.
So I would suggest you to add a 4th alternative in your document which retakes most of alternative 3 :
- Resize jpg into lower resolution so that they won't exceed 5mb each
- upload them via FTP into the specified album folder
- upload them via FTP tool from the gallery (so that GD can build the thumbnails and intermediate size if activated)
- upload the original "oversized" pictures on the same specified album folder and overwrite them all
- use the utility tool "reload the images informations about size and dimension"; so that when you rollover your cursor over the thumbnails, the correction information is shown.
Doing this will allow you to keep the original size of your pictures in the best possible quality.
And for people who are using RAW pictures like CR2 for Canon, this is what I did in my gallery :
- added "cr2" in the advanced parameters on the "text documents" to accept
- added "cr2" in the files types as a document to be opened with application\fast viewer (could be Irfanview as well i.e.)
- uploaded via FTP into the specified album folder
- uploaded via FTP tool from the gallery (GD is not used since it's not an "image" format)
Of course, there is no thumbnail created, but the download (or in download in zip file) is possible based on the name of the image.
I guess I could make my own thumbnails using the specifics in the configuration and then upload them into the same folder. Hopefully the link will be done with cr2 formats.
When a person is looking for a specific image, he knows exactly where and what to search for, since I have categorized the albums on year and month.
I think that this will also help others using DNG format for their files.
In the mean time, I use the gallery as a "backup" for my original pictures and being able to access them at any time from any place in the world.
Sorry for insisting on this topic, but I read various of forums related to pictures, and the same questions pop up all the time : RAW files (CR2, DNG, etc...), and at this point, no php based gallery is made specifically for photographers who would like to sell (or give for free for non commercial use) their original pictures.
Adding this Alternative 4 could help a few desperate people like me.
kind regards