Since your ourput will be HDV 1080i, your pictures need to be at least 1440 x 1080 non-square pixels (PAR 1.33). Since pictures acontain square pixels, the size should be at least 1.33 x 1440 = 1916 pixels width and 1080 pixels high.
Since you want to apply panning, multiply these svalues by 2. That is usually sufficient for the panning.
And for pictures with transparancy, PNG is indeed OK. For the pictures not containing transparancy, you can just leave them as .jpg.
Theo