In order to share my layout (namely "trains45") i am asked to share some jpg files, but i don't know how to proceed...

Hi Jacques

In order that any other user can see your work the way you can see it they need the original .jpg file(s) you used when texturing elements. To avoid seeing question marks instead they ask to share these files so they can have it on  their machine and link it again to the object(s) initially assigned by you. Sharing files is easy and free of cost at sharing platforms like Dropbox . Once you are a registered user there you can upload files and share with others either by inviting them by e-mail and social media platforms (sending a link) or you can use the "create a link" function in Dropbox to have it already for pasting in your clipboard (you could also just copy&paste a link). To share a link here in the forum you just paste it in the comment/answer you're doing.

Cheers

Tom

Thank you Tom, i'll try it