PhotoFiltre or Paint.NET will be great companions since they are fast, easy and powerful enough, especially when you need only to retouch, apply effects, etc. For a PhotoShop alternative, a heavy editor that will let you perform quite complex editing, you can try (what else?) GIMP.

Conversion of pictures from some format to another can be an everyday task. What worked for me the best was a right-click option: Instead of having to open some of my photo editors, it proved faster and easier to just select and right click on these pictures as I see them in their folder and convert them at once. Morz Image Converter will do just that, supporting a lot of picture formats.

Picture Resize can also be an every day task, – if you send pictures by email all the time, etc. Microsoft’s free Image Resizer powertoy will install a right click option to let you easily change one or many pictures just while you see them in their folder. It includes the options of saving them in different names or not, and of using one of 3 main default sizes or define a  custom size you might want.

Besides these programs, there exist also applications to perform single special tasks, such as optimization (reducing the quality of an image without visible changes), photo morphing, etc. You can find such utilities in the Graphics section.

See How to use PhotoFiltre, check the best free image editors, viewers and animators and the best free photo album, slide show and gallery makers.