' 图片宽度和高度
imgWidth = 100
imgHeight = 200

' 计算图像数据的大小
imgDataSize = imgWidth * imgHeight * 3

' 创建 BMP 文件头
bmpHeader = Chr(66) & Chr(77) & _
            Chr(imgDataSize And 255) & _
            Chr((imgDataSize \ 256) And 255) & _
            Chr(((imgDataSize \ 256) \ 256) And 255) & _
            Chr((((imgDataSize \ 256) \ 256) \ 256) And 255) & _
            Chr(0) & Chr(0) & Chr(0) & Chr(0) & _
            Chr(54) & Chr(0) & Chr(0) & Chr(0) & _
            Chr(40) & Chr(0) & Chr(0) & Chr(0) & _
            Chr(imgWidth And 255) & Chr((imgWidth \ 256) And 255) & _
            Chr(0) & Chr(0) & _
            Chr(imgHeight And 255) & Chr((imgHeight \ 256) And 255) & _
            Chr(0) & Chr(0) & _
            Chr(1) & Chr(0) & _
            Chr(24) & Chr(0) & _
            Chr(0) & Chr(0) & _
            Chr(0) & Chr(0) & _
            Chr(0) & Chr(0) & _
            Chr(0) & Chr(0) & _
            Chr(0) & Chr(0)

' 创建图像数据
' 这里仅是一个示例，将图像数据设置为彩虹
imageData = ""
For i = 1 To imgHeight
    For j = 1 To imgWidth
        ' BMP 中的像素数据是 BGR 格式
        imageData = imageData & Chr(i Mod 256) & Chr((i + j) Mod 256) & Chr(j Mod 256)
    Next
Next

' 将图像数据添加到 BMP 文件头后面
bmpData = bmpHeader & imageData

' 将 BMP 数据写入文件
Dim fs, outFile
Set fs = CreateObject("Scripting.FileSystemObject")
Set outFile = fs.CreateTextFile("output.bmp", True)
outFile.Write bmpData
outFile.Close

不是让你处理BMP图片，而是让你直接创建一个指定大小的BMP文件，从无到有。