HI START

From ZDoom Wiki

HI_START is a marker that is used in conjunction with HI_END. It is used to define high resolution textures that already exist and scale them according to the original texture. This is handy if you wish to create a wad which replaces the original game's textures with hi-res ones, no matter what size, without the use of typing up a HIRESTEX lump.

Note that it only works with lumps that have been identified as textures. If you try to replace a non-existent texture the image's dimensions will be used unscaled.

If you are using ZIPs as WAD replacement then you can simply use a directory called "Hires".