I'm currently running CyanogenMod 7.0.3 on my HTC Desire, and pictures copied to the SD card disappear!
If I copy an image from my computer to the phone using USB Mass Storage mode, it disappears as soon as the copy is finished.
Pictures taken with the Camera app disappear too.
Pictures copied with adb push some_pic.jpg /mnt/sdcard/DCIM/100MEDIA don't show up in the Gallery app.
.nomediafiles usingfindand found one in the card's root, but I suspect it's been recently added by a recent ROM Manager bug (I didn't take note of the file's creation date before deleting it). – Daniel Serodio Aug 16 '12 at 14:45/mnt/sdcard/.nomediaand/mnt/sdcard/DCIM/.thumbnailsand rebooted the phone, and the pictures are back in Gallery. Let's see for how long... – Daniel Serodio Aug 16 '12 at 14:59.nomediafile away :) I added this as an answer, including some closer explanation -- so you might want to check and possibly even accept it, as it solved your issue. – Izzy Aug 16 '12 at 15:33