Consistent error names in the images package

Error names in our `images` package use the singular and plural forms of "image(s)" interchangeably. We should remove "Image(s)" from the error names to avoid inconsistency and stuttering when using e.g. `images.ErrImagesNoURL`.

Start by locating the images package and the ErrImagesNoURL declaration, then review the other image-related error names for the same inconsistency. Rename the affected errors consistently and verify that references to the old names no longer remain and the package still builds.

Written by the indexing model from the issue text.