Mapmaking for Roll20

I have purchased many packs from the marketplace and want to use them to create encounter maps, taverns, city maps, etc., but to upload ALL of those many resources into roll20 seems like not a good idea.  Where is there a mapmaking program or software that is easy to use?  I am going to make the map from all the pre-purchased items that I have in the packs so what I would really like is something like drag and drop.  Perhaps that lets me resize the items as some of the packs have images that you need to make a certain size in order to see them correctly. What are people using?  I don't want to CREATE content, i just want to arrange the content I have purchased and then be able to import the final product as a map for roll20.

I use GIMP to create my maps from the purchased tile and token packages that I downloaded.

I also use GIMP, though mostly for interiors (buildings or caves). I generally use Wonderdraft for overland and town maps.  You can set up your marketplace purchases as custom assets in Wonderdraft.