Icons for the start menu and shortcuts and such are contained inside Bearsampp.exe itself.
To change those requires a few steps...

1. Create your new 128px x 128px image as a png.
2. Go to https://www.freeconvert.com/png-to-ico and convert the png to .ico.  I recommend you leave the settings at their default.
3. Go to http://www.angusj.com/resourcehacker/ and install it.
4. Within Resource Hacker open "Bearsampp.exe"
5. Following the instructions on Resource Hacker website replace the existing icons Ctrl+R
6. Compress and save

Keep in mind that modern windows can have many colors on the task bar and startup menu so the icon might be on a dark background or might be on a light background so choose your icon style appropriately.