Easy Multiple Resize Image Changing the size of many images simultaneously is very easy , you don't have to use expensive software but you can also use free software. One of them is using IrfanView image viewer. The following is how to change the size of many images simultaneously: Get IrfanView. Open image. Select Menu File-> Batch Conversions/Rename. Work as: select Batch Conversion. Output format: to specify the file type, the options section can be used to reduce image quality. Check the use advance options section. Outut directory: to save image results after processing. On the right side, select add all if you want to process many images. After everything is correct then select Start Batch. The way to change the size of many images is very easy and practical.