user_texture_idRenderer texture identifier (ImTextureID bigint), not an SFML texture object.
image_sizeDisplayed image width and height in screen pixels, independent of texture dimensions.
Read about ImTextureID here: https://github.com/ocornut/imgui/wiki/Image-Loading-and-Displaying-Examples
'uv0' and 'uv1' are texture coordinates. Read about them from the same link above.
Image() adds style.ImageBorderSize on each side. ImageButton() adds style.FramePaddingX horizontally and style.FramePaddingY vertically. Here style is the mapped view from ImGui.viewImGuiStyle(styleHandle).
ImageButton() draws a background based on regular Button() color + optionally an inner background if specified.
Implied uv0 = { x: 0, y: 0 }, uv1 = { x: 1, y: 1 }
Display an image.