I agree with John, you need at least twice the resolution in your pictures…. When exporting from Powerpoint, the default is NOT good enough, there are settings you need to make to Powerpoint, forcing it to export higher resolution, for example…
To answer your question about sizing– what I do is use Irfanview (free picture editor) to crop the picture, or add borders to the picture, so that it is the desired ration. For example in SD videos for traditional TV screens the screen ratio is 1.3545. So, for example if the picture is twice as wide as it is tall then I decide, should I crop it down from 2.00 to 1.3545? or add borders to top and bottom, or a combination. Usually I do NOT stretch but sometimes I do that. Irfanview can apply your decision to unlimited numbers of pictures.. it has a great batch processor… but I think Vegas can do it, but I don’t know how.