If you use the GIMP, and want to resize a LOT of pictures in one go… install gimp-plugin-registry then follow these instructions:
Start Gimp. Access the Batch file plugin via the Filters > Batch > Batch Process menu
If you want to resize and output to a different directory, make sure you change the settings on ALL 4 of the following tabs, BEFORE selecting ‘Start’ (by default it will output to the same directory and convert to BMP)
Input tab - Select files to add to the batch
Resize tab - Select the size you want (and enable)
Output tab - Select file type (BMP, JPG etc.)
Rename tab - Select dir (directory for output)
It can do a lot more than just resize… Turn, Blur, Colour, Resize, Crop, Sharpen and Rename… or any combination of these.