Gimp doesn’t have built in batch resizing functionality, but it’s pretty simple to add it via a plug-in called DBP (David’s Batch Processor).

1. Download Windows version of DPB from here.
   (For Linux version check DPB home page).
2. Extract dbp.exe file and copy to: C:\Program Files\GIMP-2.0\lib\gimp\2.0\plug-ins
3. Open Gimp and open Filters > Batch Process
   
4. In David’s Batch Processor dialog:
   • Input Tab: Add files you want to process
   • Resize Tab: Configure resizing settings
   • Rename Tab: Choose where resized pictures will be saved.
   • Output Tab: Select and configure image format.

   

5. Click on Start to begin batch resizing.