WinHelp graphics must be in WMF or BMP format. Graphics are viewed at screen resolution, typically 96 DPI. Normally you want to use WMF, because WMF graphics have a much sharper image than BMP graphics. However, the WMF format has some drawbacks; see §29.1.3 .
For very large graphics, BMP can be a better choice; see §18.104.22.168.3 .
WinHelp 4 allows 256-color bitmaps; the added space for 256 colors is relatively small, and the graphics usually look better than with 16 colors. WinHelp 3 allows only 16-color bitmaps (unless you use add-on DLLs). For any WinHelp use, 24-bit color ("true color") is a very bad idea; it often crashes the Help Compiler during processing.