ResizeIt allows you to change the size of multiple images simultaneously. It can also convert file formats at the same time. It supports multi core and processes multiple images very fast.

When the app is launched the file open dialog will be shown, so please choose photo files to be processed, then the app shows setting window. Dragging and dropping photo files can be accepted too. Then just input setting and hit Resize button and it's done.
The setting can be saved as preset.
If the aspect ratio have to be kept, just input 0 to height or width then ResizeIt automatically calculate the appropriate value and resize. For example input H:500 W:0, ResizeIt calculate width to keep aspect ratio.
Please refer to manual about other details. Manual can be shown by choose ResizeIt>Manual menu when setting window is shown.

If you like this software, I'd like to ask to make small donation to help effort for the Japan earthquake and tsunami disaster.