Does anyone have any suggestions for a concrete image for "increase"? Frame 501.

For 501 "Formerly", I use the artist formerly known as prince. I imagine He is wearing a crazy outfit with horns and sun (over his midsection) dancing in a rice field. (I think dingomick uses prince too but describes it differently).
Well I just happen to hate (see #626) prince, so for #502 "increase" i imagine dumping a load of dirt on top of prince in the rice field. Unfortunately though, there wasn't enough soil to completely cover him so his head is sticking out doing a seamy prince head dance so I yell to the dump truck driver "Increase it! Increase it! we need to increase the soil!" or some such nonsense.

I read somewhere that it is thought to be a pictogram of a rice cooker with steam at the top given the notion of thing increasing/building up.

I think it was in Henshall's "A Guide to Remembering Japanese Characters"

The image that has always stuck with me is a Monster that was FORMERLY a good guy, now just HORNS, growing out an exposed BRAIN, with a TONGUE WAGGING MOUTH drooling below. Got it from KanjiCan stories.

Increase is when you add the Dirt (soil) primitive, IIRC.

Thanks for the suggestions. BTW: in my book 501 is "formerly" as a character, but it is "increase" as a primative. 502 is "increase" as a full character.

Do the more recent versions have this?

Yes, it's like that in my 5th edition. Frankly, I find it a bit confusing. That's probably the thing I dislike most about RTK: taking part of a kanji as a primitive and using the same name for both. For ages I thought the primitive for 'head' was the actual kanji for 'head'.

For "formerly" I picture a "Gift Box" on the top. Maybe someone forgot to buy a present to his wife, or you see someone's else buying a present to the girl you like.

Or if you want to go the anime route, for increase the primitive or formerly, I use Takkun (Naota) from FLCL. The horns sprout out of his brain, and his tongue is wagging as he screams. (Also helps to keep the order from top to bottom.) So he's the kid formerly known as Takkun. And if you've seen the anime, you can just try to remember when the horns sticking out of his head starts emerging from his brain as giant robots. (Yeah, it was a weird anime.) And if you dump soil on Takkun, it'll just make him mad, so the number of robots coming out will just increase.