Translatewiki handout material
The original image can be created in an editable image format that supports layers. Then you can create a copy of it in a usual image format like png and jpg. If you preserve the original image files you can edit them at any time and create a new copy in png or jpg and so on.
Yes, but you would still want the original uploaded together with the end formats. I don't think that's currently possible with anything other than SVG.
I think for the images (logos) we can go with svg or psd - both are editable with free software (gimp/inkscape). For the document we should use a doc or odt (using another format to design a leaflet is sort of PITA with manual margin/padding calculation) and save the final version in pdf, png and jpg - one of these works everywhere.
A total different approach to this would be using google docs to host it once the basic layout is done (never tried it so don't know what happens to the columns when uploading there).