You give spacing: 0 in LazyVGrid for vertical spacing, and spacing: 0 in GridItem for horizontal spacing.
Here is a demo. Tested with Xcode 12 / iOS 14

struct TestImagesInGrid: View {
@State private var imageNames: [String]
private let threeColumnGrid = [
GridItem(.flexible(minimum: 40), spacing: 0),
GridItem(.flexible(minimum: 40), spacing: 0),
GridItem(.flexible(minimum: 40), spacing: 0),
]
init() {
_imageNames = State(initialValue: (0..<8).map { _ in
"image_\(Int.random(in: 1...3))"
})
}
var body: some View {
LazyVGrid(columns: threeColumnGrid, alignment: .leading, spacing: 0) {
ForEach(imageNames.indices) { i in
Image(imageNames[i]).resizable()
.aspectRatio(1, contentMode: .fill)
.border(Color.black)
}
}
}
}