They're more noticeable when in maximize windows:
2 Answers
These borders are part of the default Windows 7 windows appearance. Eithe set your border to 0 pixels width or use an alternative style.
Both may require hacks, and will not be limited to just IrfanView
As of Version 4.60 (Release date: 2022-03-18) there is a shortcut that will permit "thin" borders, in a way similar to the original request:
New hotkey: ALT + SHIFT + B: Show thin or normal border (current session only)