aside xnview, i have used most other excellent image viewers on windows platform. well, i'm attracted by xnview's speed and interface. it's really a good job!
yet, some other apps provide a built-in painting toolkit. a screenshot of IrfanView below:

I'd say, it's such a handy toolbox that should be considered.
two reasons why it's a must-have:
1. though a image viewer it is, it provides varies of image processings. so a simple plain toolkit is just one part of it. it helps user easily make comments on a picture. such as, add texts(yes i know it's already in), draw lines, boxes, arrows, etc.
2. xnview has a built-in screen capture function, so what next to capturing? yeah, of course some modifications will make a picture more lively, especially when the picture is used to desribe a process or a problem.
YES, xnview HAS an addon, working as a filter, which deals with basic paintings. but consider it's so weak that even M$ paint can simply defeat it. and... it's external, when you use the "paint filter", you cannot make other adjustments. the only thing different from an external image editor is it doesn't have a taskbar tooltip...
so i hope you can take it to your consideration. and if you accpect my suggestion, i can help draw outlines of this feature.
TIA.