I have about half a gig of large compressed TIFF files we'd like to load onto an SD card and view inside a Pocket PC. Good in theory, but the files are so large when uncompressed we can't reliably open them.

An average file is about 10000x7000 pixels, about 8 or 9 MB uncompressed, and somehow they've been compressed to about 300 or 400 KB each.

What I need to do is shrink the file resolution (half should do, to 5000x3500) and then put the smaller files in the Pocket PC. Except even if I shrink the files to 2500x1750, I can't find a way to save them such that the file size is less than about 500 KB. 500 KB is too big - I can no longer fit everything on a single SD card. I've tried GIF, PNG and all the TIFF variants which IrfanView supports. JPEG isn't acceptable; they're line drawings and detail is vital.

Does anyone know of a really good lossless compression algorithm for TIFF files, and a program which implements it? A suitable algorithm obviously exists and was used to create the original files, but I have no idea what it is.