There aren't any restrictions about being in on-line galleries prior to publication. If you're accepted for pub we will ask you to pull it from any galleries it's in from time of notification of acceptance until the end of the publication month, then you can re-post your pictures.
Send in a 300 ppi image, roughly 500x500 pixels. If it's a two-pager, it can be 500x1000 pixels.
Here's a link to a thread in the magazine forum that has additional information:
http://scrapstreet.com/MessageBoard/...p?topic=9268.0
Please let me know if you have any other questions!
Janice