20001: Layer uses a picture symbol with a reference scale
A layer in your map uses a picture fill symbol with a reference scale. Your map will correctly display picture fill symbols at a fixed (reference) map scale. This portrayal of fill symbols will differ slightly from the standard ArcMap display because there are slight differences in picture fill symbol rendering within ArcMap and in basemap layers or optimized map services.
This warning message is simply telling you that fill symbols in your map service display will differ slightly from your ArcMap data frame's display. Map rendering in basemap layers and optimized map service is correct. However, it will differ slightly from your standard ArcMap display.
Fill symbols are typically used to fill polygons. Fill symbols can be of a number of types—for example, a solid color, line hatching, a gradient color, an icon, or a picture (bitmap). This display difference only occurs when using picture fill symbols. See Creating fill symbols for more information on alternative fill options that you can use.
This display difference only occurs when you have set a reference scale, which fixes the symbol size to draw at the proper height and width (typically expressed in points) at the reference map scale. This will cause your fill symbols to become larger as you zoom in from the reference scale and to shrink as you zoom out. It's important to recognize this behavior because it is not typically what Web map users want in their online, multiscale basemaps. It is likely that you will want to disable this reference scale behavior. See Working with reference map scales for more information.