Problem: Transparent background color in ArcIMS raster symbols are not supported in ArcGIS
If a feature service layer has been authored to draw its features using a transparent background GIF raster symbol, ArcGIS will draw the raster symbol with a gray background.
ArcGIS does not support transparency for GIF images.
Solution or Workaround
Create a new version of the symbol as a glyph in a TrueType font, or create a new raster symbol that doesn't have a transparent background.