photoshop CS2 greyscale dot gain / brightness lossiness

Photoshop CS2 comes with colour management turned on by default, which means that for every image saved to an image format that doesn't support colour management, the image is processed/converted to the working profile on save. the next time that image is reloaded, photoshop doesn't remember that it's already converted the colour profile, and does it again. this means any saving to a format that doesn't support embedded profiles (e.g. BMP) is inherently lossy.

disable colour management (edit > colour settings) to fix.

Photoshop CS2 jpeg bloat & incompatibilities

Even with colour profile embedding off, jpeg's saved from CS2 contain about 20KB of bloat that makes them wholly incompatible with Acrobat. ironic, no? To fix, run lossless JPEG transform in null/clean mode from irfanview or something comparable.