You don't need to reset your DPI...That's a reference point only, so you know what the largest size of enlargement is...
I use "Faststone Photo Resizer - converter 2.5 (free)...You can select what the long side will be..(For SC..600px) and check "Keep aspect ratio"..
And there is a Quality setting under "settings" on the right lower side (Blue area)...and more settings under "Advanced" for everything else..including Text Copyright or Graphic watermarks (Could be a copyright graphic file)
BTW...It can save the EXIF data too...Some forums want the EXIF data preserved for proof of the Camera and shoot date. Like for contests.
Link below
FastStone Image Viewer, Screen Capture, Photo Resizer ...