The hieroglyphs should have a transparent background. This can be done easily without increasing the size of the png file, other than the addition of an indexing table. Without this, text on other colored backgrounds just looks wrong. This is an easy fix: I've done it multiple times on Wikipedia. If you want, I'll gladly convert the glyphs for you if you'll tell me where to upload them. --trlkly 14:33, 25 June 2012 (UTC)
Topic on Extension talk:WikiHiero
For documentation: it's reported as Bug 31736 - Use transparent images for hiero.