Simplest Image Converter

If you are looking for an easy way to batch convert your images or just resize them, you should use a specialized application, like Simplest Image Converter.

Simplistic interface

The utility's only window allows you to make all the picture changes that are possible with this program. You can import the images that you want to resize or convert and choose the folder where to output them. The files are saved in a separate subfolder named Converted, and not in the root folder that you actually selected.
A downside of the interface lies in the fact that you can't remove multiple files at once from the list. You have to click each entry and then press the Remove button.

Basic picture resizer

Aside from the conversion feature, the utility is able to batch change the width and height of the output files. Sadly, there is no way to lock the aspect ratio, so you either have to figure out the correct resize values or end up with, possibly, distorted images.
Alternatively, if you just want to convert the files, you can enable the option to keep the original width and height.
Furthermore, Simplest Image Converter can import, as well as export, commonly used picture file types, such as JPG, PNG, TIFF, GIF, BMP and ICO. This means that the vast majority of your files will probably be supported.

Rudimentary image converter

If you want more options, like the ability to preview the files before converting or the ability to lock the aspect ratio when changing the picture's dimensions, you might want to look elsewhere.
In conclusion, this application functions as you would expect from its name. Moreover, the small size and the fact that it does not require installation means you can carry it around on a USB flash drive.