When you insert the image, it should return you an InlineShape, which you can modify:
Word.Application app = new Word.Application();
var doc = app.Documents.Open(@"C:UsersSomeUserNameDesktopDoc1.docx");
var shape = doc.Bookmarks["PicHere"].Range.InlineShapes.AddPicture(@"C:UsersSomePicturePictures1234.JPG", false, true);
shape.Width = 150;
shape.Height = 150;
app.Visible = true;
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