Commercial Extentions for us in Joomla Theme

Dear Folks,

Q1) I am submitting my Joomla theme to themeforest. so I am using PAID Extentions in my theme and I am authorized to use in themeforest. But how i mention during Submission that I am authorized to use PAID extention in my theme,?

Q2) For Preview Images, I created Image size 594* 1000 , is it correct ?