Every Wc3 mapmaker should have a MPQ-Viewer/Extractor. Me for instance uses
Magos Model Editor that includes one. Wc3 has 4 MPQs, that are data packages, one RoC package, one TFT, one for latest Patch and the last for local varying data such like different language versions. TFT files with the same name/path overlay RoC files, Patch overlays TFT (or RoC, in case you have no TFT), when using current patch.
So there is no ReplaceableTextures\Splats\Splat01Mature.blp file in War3Patch.mpq but one in War3x.mpq (TFT). So this is the currently used one. Extract this file, blp is the standard graphic file type Blizzard uses in Wc3. It's their own custom format, so you need a converter. The
Image Extractor can do the job for example. This tool also has a MPQ browser but only displays 2d art. Convert the file to tga, edit it with a 2d art program that supports tga like photoshop, save and convert back to blp. Then import the file in your map setting its path to ReplaceableTextures\Splats\Splat01Mature.blp to overshadow once again (the map has even higher priority). You cannot replace all files though but it works for most graphics.
Ps: I wrote Viewer/Extractor above, there are also MPQ Editors, but you are not allowed to change the game's MPQs.