Once you are in google images or do a search using images as an option, search for icons as a type.  For example,” folder”, then search using an explicit 256x256.  Do not use a file size as that doesn’t reflect image size.  These are two different things.

 I have included two images.

1. The first one shows a search for “folder”.  This shows size first as “any size” checked.
2. Click on exactly.  It will immediately bring a size dialog box.
3. Enter 256 and 256.  That will search for what you need.

As a note, if you plan to get a smaller 4k (UHD) monitor you may want to use 512x512 instead.  I just got a 4 k monitor and wished I had used 512x512.

 I looks like it isn't possible to insert images into stardock.  The insert icon only inserts urls which is useless.  I will describe it as follows:

Once you get the size dialog box, ignore file size and select image size.  Select search tools followed by size.  You should get a dialog box whit "exactly" at the bottom.  Select that box and put in 256 px for width and height and then gol