Aesthetics is a big issue, it's just plain tough to make a good-looking avatar sometimes. I'm a grunt-modder for an online game that I play (Medal of Honor) which uses avatars as an ID system for team-mates. I've tried more than a few things and very few of them look really good.
Relevance is also an issue, but less so than appearance. Nevertheless, if anyone has any suggestions for relevant images for use as avatars, why not post your thoughts? This way we can all get an idea of what others are interested in, what their preferences are and hopefully come up with a happy medium.
Here's some of my suggestions;
Simple pics of modeling subjects; tanks, a/c, cars etc.
Manufacturers' logos (Ford, Chevy, Grumman, Curtiss - copyright issues here, of course!)
Modeling-related objects; paintbrushes, X-actos and the like
Markings avatars;
For Aircraft - maybe US stars and bars, a German balkankreuz, British roundel etc. Simple unit insignia such as 8th AF, 9th AF and the like. Rescue arrows, ejection seat warnings etc.
For Cars - flames, or checkerboards.
For Military Vehicles - a simple white star on an OD background (with or without surround), unit insignia.
And let's not forget ships and sci-fi and anything else!
Historical avatars; many possibilities here, from simple photos to stylized graphics of almost any sort you could imagine and related to any modeling genre.
By no means is this intended to be an all-inclusive list. It's simply meant to get some gears turning. Sources for any avatars deemed appropriate, of course, would be another matter! If anyone has ideas on this issue, I'm sure that we'd all like to hear them.
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