Is it possible to set image attachments as...
 

1000 pixels wide as thumbnails with 95% (or full) qality) and when clikcing on them having them in full resolutiion (native resolution)?And also list them as 1 per row I beleive makes then sense.

Allso, do I NEED to rebuild all thumbails after this and do I need to close down the forum for that?

Thank you in advance.