I had a picture book published in 2006 which is now out of print and the rights have been returned to me. Is it okay to submit this to other publishers, and if yes, then when is it okay to do this? And if I can submit this do I mention its previous publication? Thanks for your help.Yes, you mention its previous publication. The editor will find out anyway when she does her acquisition research, and she will be pissed if you've failed to tell her this yourself.
Here's the thing about books that have gone out of print: most of them are out of print for a very, very good reason. It may be a painful reason, and it may be a reason that makes no sense to you, but it is still a GOOD reason: NOT ENOUGH PEOPLE WERE WILLING TO BUY IT.
If this is the reason that your book is out of print, then no publisher is going to bring it back into print within a couple of decades of its original publication. If this is not the reason your book is out of print, then be very clear in your submission to other publishers about what you think the real reason is. Be clear, and be convincing, because you're fighting a counter argument from the market, and publishers listen to the market.