What, exactly, makes them complain about poor picture quality?
If you're shooting an uncalibrated projector toward an arbitrary projection surface, you've got a lot of things working against you. First of all, your color configuration is liable to be all over the place. Secondly, you're going to be at the mercy and the reflectivity of your wall.
I'd recommend a couple of things, assuming that the photos being projected look good on a a computer or on a mobile device.
Calibrate your projector. This can be done by hand, but it's really worthwhile to invest in something like an X-Rite ColorMunki Display. This will quickly, automatically, and reliably measure the color performance of your projection system and create a software profile to correct for any deficiencies. If your club has a shared computer lab, you can use the same device to calibrate all of the displays in the lab.
Also, be sure to set the projecting computer to use the projector's native resolution. If your projector is operating at 1024x768, but your computer's output is set to 1280x800, you're going to introduce all kinds of weird effects as the image is scaled and stretched to match the projector.
If you're regularly projecting against the same wall all the time, look into buying "screen paint" or something similar. This is usually a neutral grey in color and may have additives to boost reflectivity. You'll find more info about this in DIY home theater forums. Note that you do not want to project against a black surface if you can help it.
If you're not regularly projecting against the same wall, you might consider building a DIY projector screen. This could be as simple as PVC and a white bed sheet, or you could look into a more suitable screen surface. This can be built DIY for $100 or less, depending on your materials, and is something you can easily build in your garage in an afternoon.