Here's the ideal for me ("the ideal"--not what I expect or demand, but--hey, we're DIYing this Old School Gaming thing so why not start doing things a different way?)...
Rather than a writer going "Hey, I'm gonna write an adventure about Giant Worms From Space, then once it's done I'll send it to this artist that I think would draw awesome worms and see if I can get him/her to illustrate it."
Maybe try..."Hey I'm gonna write an adventure about Giant Worms from Space. I'm gonna tell this artist who I think would draw awesome worms -right now- and commission him/her to send me a picture of some Giant Worms From Space doing something right now and then maybe I'll get some ideas, and I'll use the picture in the adventure. Then wherever I get with writing on that, I show the artist, and then I'll ask for another picture, and then maybe I'll get some ideas from that, and we'll go back and forth and make an awesome adventure."
All OSR writers are DMs, most artists are, too. If their work seems interesting and it seems like they would be right for your project, it probably means they have ideas about that thing. You can't actually draw a Giant Worm From Space without thinking about one at least a little.
And yeah, that's maybe harder, you have to commission the picture before you're even sure the thing's written, but we should be trying new crazy ways of doing things. For the DIY D&D artists, who--let's face it--are not getting paid a lot, being involved in the project at this level makes it much more fun and exciting, and may make them more willing to get involved and put work in. It's their baby too.
If an artist is working for you, chances are s/he would like to one day hold up whatever s/he makes as a portfolio piece to attract future clients. That means s/he'd really like to do something s/he thinks is interesting and is engaged in.
For further reading on the subject of artists having good ideas to contribute to media involving both words and pictures, see:
Stan Lee and Jack Kirby
d100 Villain Escapes for Marvel
3 hours ago