Yes...always resize all at once. I find it best, though not always easy to...make the picture the size I want it to be for its use. The picture format will play a role in this to. I like lossless formats depending on the program used to create the pic. such as .ai, .psd, .tif, bla bla bla just stay away from editing the same .jpeg or .jpg over and over again you will end up with bad quality and bad color as well. and if for some reason you do have to edit you reaized pic again.....I would say don't, start from the original pic....