ImageGlass is a really lightweight, basic image viewer with decent range of supported formats, pleasant user interface, and a nice feature set that satisfies all your daily needs.
Ranked in these QuestionsQuestion Ranking
Pro Open source
As long as you have enough skill you can mold it to your liking.
Pro It handles animated GIF files really well
Animated GIF files (let it be transparent or regular) are played seamlessly even if you zoom in and out.
Pro Portable version available
If you don't want to or you can't install ImageGlass, you can go with the portable.
Con Does not support APNG
It does not support the Animated Portable Network Graphic (APNG) format yet. And it is a con worth considering.
Con Color picker doesn't have an X [close]
And can only be closed with the menu options or by ctrl+shift+k.
Con No editing options
The program lacks any kind of image manipulation or batch processing features like Irfanview or XnView has. If you're an advanced user, this will be the biggest con against this software.